Reddit reviews EZ 101 Tankless Water Heater - Propane (LPG) - Portable - Battery Powered Ignition - Camping - RV - FREE UPGRADE

What about tankless? They seem much more efficient than the standard kind of water heater, even if you're wanting to heat already-warm water.

Yeah I have all of my plumbing inside as well, except for the black water and grey water tank.

So the hot water heater....I wanted an on demand unit. I originally bought this unit https://www.amazon.com/EZ-101-Tankless-Water-Heater/dp/B003327HDK. I started realizing I just didn't have anywhere to put it. You may have better luck cause you are starting totally from scratch it looks like, but my van already had walls up when I bought it. I still have this unit, and I sat on it forever trying to figure out where to put it. It just wasn't going to work. I couldn't mount it easily and run the vent piping.It also draws its combustion air from the space it is mounted in which could be a concern depending upon your use. I was just planning on opening the roof top vent and securing it while sleeping. If you are in the SF bay area I will sell it to you cheap. 50 bucks and its yours :)

I realized I had to buy a different unit, and it ended up being very expensive. I bought https://www.amazon.com/Suburban-5132A-Demand-Water-Heater/dp/B013002JD2 I found it for about 600 on Dyers RV. I have it installed. I haven't used it yet, but it was very easy to install I just had to drill a 4 inch hole in the side of the van and it does intake and exhaust through a hole there with a concentric vent so there is no fire hazard because the exhaust comes out of the inside of the concentric circle. It requires very small clearances. 1/4" on the top and sides so it saves a lot of space. It is definitely a big budget item, but it will be worth it in terms of space I believe.