MtnDon wrote:I might consider using a different water pump if the noise from teh RV pump was too much
The typical RV water pump uses a lot of power... like 7.5 amps @ 12 VDC for a Shurflo 4008. And they are noisy. I would suggest a serious look at a small "solar" pump such asthis.
I don't know if this is a concern or not, but RV pumps are built with the use of pumping cold water in mind. The Shurflo specs do not list an operating temperature range. The circulation pump I linked to has a rating of up to 212 F. And it is quiet. I don't see amps listed but the one I have is less than 24 watts; about 18 watts, IIRC.
My design is theoretical, since I don't have the water heater and I rarely camp when I "need" a heater. But after further consideration for the " on demand" version I think the solar pump is a better idea. I would probably install my heater core with some quick connect fittings, either under the cabin or through the bulk head. The water heater would probably hang from the side of the TD. A small ice chest could provide the reservoir for the system. The insulation would increase the efficiency. One question to consider is how big the reservoir and volume of water need to be. Odds are that some systems will allow the water to drain back into the bucket, so there would be a change in water level when the system was running. The only risk I can come up with is that if the water level drops too low the pump will run dry, but could continue to run.
Tom