Having mucked around with all sorts of schemes in various Houses and Trailers, I always come back to the 'Physics 101' fundamentals of this Thread question.
1. The Water volume has to be stored somewhere. Wherever that is takes up the same volume. I would tee on a 2nd small cold Water Tank if the root need is more Water on board.
2. Having seen them Worldwide during Biz Travels, I got a Demand Heater from 'Grainger' [240 VAC - 40 Amp Breaker] and installed it in one of our Houses. Worked well with some tweaking. A mandatory Gauge on the main Water Line from the Cisterns I installed allowed me to watch Shower flow precisely. With an el cheapo low flow Shower Head ['Water Pik'?] set for 'needle'-like jets of Water, our use was 3/4 Gallon/min. with a Demand Heater for a comfy Shower with Water left flowing. Flow never varied. Well Pump pressure setting at the Pressure Tank was the Factory setting of 50 PSI Pump Off - 30 PSI Pump on. The Pressure Tank internal bladder does it's job of equalizing out flow between Well Pump power cycles. I intentionally chose the largest Pressure Tank I could source locally: a 28 Gallon one. The other Pressure Tank 'half' is pressurized Air, so this Tank is just under the size of a 55 Gallon Drum. This capacity causes the Well Pump to cycle much less.
There's a small Washer included in some low flow Shower Heads. I've modified this Washer, or drilled a smaller hole in a new Washer, to get the flow rate desired.
3. I use only Ball Valves in all projects. They allow you to really tweak settings; critical to obtaining repeatable results from Demand WHs.
4. Experimentally, I would throttle down cold Water going into the Ecco and confirm what the lowest possible flow is before it shuts off. I also would dial down Ecco Water temp to the lowest setting to see what the bottom line is on Ecco flow rate and output temp.
5. We Solar types have used 'Mixing Valves' for decades on the output of Solar-heated hot Water. This mixes cold and hot Water at the WH output to prevent scalding, and ensures minimum hot water usage. You could install a Mixing Valve IF the Ecco hot water 'part' of this flow wasn't so low that the Ecco shuts off internally.
The 'new' RV thing is to crank up a standard 6 Gallon WH and use a Mixing Valve on the output. It's then marketed as having the supply capability of a ~10 Gallon WH. Heating and holding 6 Gallons of 'super' heated Water is wasteful, but that's not an issue on the Big Rigs.
6. Energy is energy. To use energy, and store that hot Water in even a super-Insulated Tank [which is how I would do it] means you're using energy amount 'x', and then wasting it while storing that Water. Or, you will have wasted energy in 'overheating' Water not used in the Shower in real time.
I'm a huge fan of measurements [Voltmeter, etc.]. I have 2 or 3 Radio Shack Indoor/Outdoor Thermometers, and use them religiously. The Outdoor Probe can monitor, say, Fridge/Cooler temp. It's also handy for measuring hot Water temp out of the Shower Head while you make adjustments. Then, if you're camping in a cold location and the Showers aren't cutting it, you can adjust the WH temp and measure/quantify what's really happening.
7. I use tricks like using a clean 1 Gallon Water jug with part of the upper side cut out [but the Jug 'Handle' left intact for carrying]. Run preheat Water into that Jug, and use it to flush the Toilet as many RVers do, or for washing Dishes, etc.. Obviously, keeping Plumbing runs short and direct helps.
8. The Shur Flo 2.8 GPM Pump is a real, freeze-proof work horse. It should be possible to adjust the Ecco to work with that Pump and get repeatable, nice Shower results. Their >5 GPM Pump is getting really bad RV Press as unreliable. Consider that 2.8 GPM is >1/2 a 5 Gallon Paint Bucket.
The 'Accumulator Tanks' linked below act identically to the mandatory Pressure Tank used on any Well-based Water System. The internal Bladder, pushing against internal Air, 'stores' pressure. More importantly, you can really dial down Water flow through the Demand Heater to the minimum allowed by WH design. Note, too, that Water in the Accumulator Tank will temper up to ambient temp, and 'pre-warm' cold Water, as you seem to want to do.
There's some nice 2 Gallon Pressure [Accumulator] Tanks at the Big Box Stores for Home Systems that would work fine in a Trailer. The Trailer Pump would run longer when on, but stay off longer once the integral Pump Pressure Switch shuts off. More importantly, these Accumulator/Pressure Tanks even out flow through the Ecco WH.
9. Some Solar folks install 'Tempering Tanks' to preheat incoming Water to room [Trailer Interior] temp. One fellow made a brilliant Tank of paralleled pieces of 4" or larger PVC Pipe. That idea could be deployed vertically or horizontally; say, under a Bench Seat, or in a Cabinet. If tempering Water up to a warmer temp passively is the intent, there are creative solutions that are not Tanks that have to consume valuable space.
10. Water will also reach ambient temp wherever you're camping. You will wind up heating Water likely warmer than what comes out of a Well anyway. Also note that Ecco, and other Demand Heaters, sometimes don't like input Water that is much above cool or cold. This is because their designs plan on adding minimum BTUs 'x' to incoming Water. I would jury rig up an Experiment first. It will answer your questions w/o guesswork.
By experimentation, I learned in my '83 Avion Trailer to put the 6 Gallon 'Atwood' WH temp setting to ~1/3rd of the way up. I listened for the WH Flame to go out, and turned off the WH. I washed Dishes first with the hotter Water, and then Showered 'Military style' using mostly hot/warm Water with very little cold Water mixed in. This saved Propane, and made cold Water temp irrelevant, since I wasn't mixing in much cold Water.
8 - 10 Gallon Showers are unthinkable to Solar and off-Grid folks [whether Boondocked Houses on 'Wheels' are involved or not]. So, that Water usage or waste is not a realistic concern in my experience [see above]. You should be able to avoid that scenario with the ideas above IF this time-tested sort of solution floats your Boat. You'll also save money, and keep Plumbing Components [and, thus, Road repairs of them] to a minimum. With a Ball Valve installed, you can dial down flow coming out of the Shur Flo Pump. This takes out of the equation the requirement to have folks manually dial down Shower Flow, or other Water usage. I'm way into foolproofing stuff. The Japanese call it 'poke yoke' ['mistake-proofing']. They use such thinking throughout their Manufacturing to the 'nth' degree.
Put a single Valve at the low point of sloped Plumbing. This makes Winterizing a cakewalk. A Hose Fitting on that Drain Valve means you could save or direct that Water.
Accumulator TanksCaveats ~
1. I'm on a Broadband Aircard Modem, so I can't review your Build pix w/o eating up valuable Data time.
2. I don't know the minimum flow rate on the Ecco; I haven't reviewed it for a while. If it requires several [wasteful] Gallons/min. of minimum flow to heat, then I better understand where you're coming from...
by Mister Ed » Sun Mar 18, 2012 8:11 pm 
