On the weight thing in general, it sounds like you'll be about where we ended up on our build. The frame I found was too heavy (500#+ as used, 4" C channel) but it's the starting point in the build & the foundation, so the initial weight is good in the long run & on rougher surfaces. You don't want a frame failure if you're out in the boonies! A frame weighing closer to 350# would be fine for your build.
A single battery should be fine (group 24 or 27 is ample) for your needs, as a fan doesn't draw much at all & lights being mostly LED's now are negligible loads. We run a 100 watt solar panel & group 27 battery & never run out of power. A smaller battery & panel would have been fine to run all the crap we do at times (TV, Bluray, soundbar, stereo, phone charging ports). We've charged stuff for other people as well, with power to spare, oink, oink on weight & power both. Less weight is better!
Our rolling weight ended up at 1660#, but that was my fault for getting a bit carried away with the build over the course of 2 years. I ended up putting in a 3500# axle with 10" brakes recently, but towed for 12,000 miles without brakes or a weight on the scales. It wasn't until we actually weighed this thing that I got a bit squeamish about stops. We tow with a 2007 Nissan Frontier with the 4 cylinder engine (1/2 ton) & it's fine, except on big hills & mountains. We could use a bit more torque. With the 2000# idler axle, there was beginning to be a small amount of inner tread wear due to the weight (measured at 0.06" between the inner & outer tread) with that many miles of towing, so not really too terrible at all. The tire wear led to a trip to the scales, a look at rolling weight led to a different axle with brakes. If you go with a 3500# axle & brakes, you won't ever have to worry about stopping, mountains, potholes or in-town stops in traffic.
Just saying that you can safely build lighter than 500# for a frame, shed a few pounds in how it's fitted out & made & still have a strong build which is durable. If it comes in at less than 1400# then it will be easier to tow, pull, run down rough roads & still get the job done. It will be easier on the tow vehicle transmission temps in summer, brakes, etc. as well. A skeleton frame & thin skins for surface tension are all you really need & a torsion box has its own strength which will transfer loads throughout the build once it's rolling.
