I've been working on a very similar project, only using an 8' above ground pool (650 gal) as the camping hot tub. They're relatively cheap, easy to transport, and quick to set up. My first attempt to heat it was using 50' of 1/2" dia coil placed directly in the fire, as is shown here, and I ran water through it using a 1/6 hp submersible pump run off of an inverter connected to my car. It took a loooooong time to heat, probably 7 hours, and we went through a tremendous amount of wood. It was fun, but not practical anywhere that you'd need to pay for wood. $$$!! We were lucky that there was an abundance due to a landslide in the area a few years before. My next attempt was using a homemade woodgas stove with a heat exchanger just above the combustion zone. My goal is to heat the tub using pellets, which are cheap and easy to deal with. Details of that build are irrelevant, as it worked great in a batch type burn, but would fill with charcoal for the longer burns required to heat this much water, and turned out to be too annoying to deal with. Refilling it was a mess and would drastically effect the flame, very inconsistent temperatures resulted, and intermittent big black clouds of smokey unburnt fuel would be lost out the chimney. My third attempt was using a rocket stove with a heat exchanger on top. With a 6" diameter burn chamber, 4 x 50'x 1/4" diameter copper coils in the heat exchanger, everything veeeeery well insulated, the system converts about 40% of the potential energy of the pellets into water heat in the pool. I think it outputs a real water heating value of about 23,000 Btu/hr into the pool. Which i think is pretty decent for a portable homebuilt system. It resulted in a 9 hr heat time, and only costs $1/hr to run, and produces almost no smoke so you know you're not wasting any of your fuel. 9 hrs is still waaaay too long though, that's a whole wasted day. (A side note: there is massive restriction in 1/4" copper tubes, even having 4 plumbed in parallel caused a big reduction in flow. I wouldn't recommend going this small.) The rocket stove was well behaved though, and the concept seems to work well. To get the heat up time down to a passable 2-3 hr range, I think you'd need a rocket stove with about 400% of the output of my 6" diameter system. I haven't built or testing the next prototype yet, but I'm thinking it should be a rocket stove with a 12" diameter burn chamber, which according to simple math should get me to 400% increased output. I haven't figured out how to build this yet, so can't offer any advice there, but according to my tests, and the math, this seems to be the size that would be practical for this application. I hope someone out there find this info handy, it's taken months of meddling!