Re:Alternatives to a hot water heater?
Not to mention the freeze factor.
We never put our fresh tanks on the exterior of trucks for the freeze reason. Grey water we don't care as it has a 1 1/2" pipe and the chances of a full freeze that would result in cracking of a pipe is non existent. Fresh however we run with 1/2" pipe, higher chance of it freezing.
As far as hot water heaters that are gravity fed:
Only ones I have ever seen are the ones I have built. Look at most of the hot dog push carts out there. A lot of them have water heaters. Essentially it is just a stainless compartment just above the sink area with a 2 burner camp style gas stove under it heating the compartment, which then rolls downhill to the sink. Simple but efficient.