Back in my tent camping days in the 50s and 60s I made up some metal pipes hanging just above the camp fire then added a small water tank and routed the hot water hose to a modified car radiator sitting just inside the tent.
Worked like gang busters but you had to keep a bonfire going all the time... I never did get the temperature to be adjustable as it was always way too much heat hehe... Did work however...
The small hot water tank was just a holding thing for water. If I added a small water pump I could have adjusted the heat pretty easy. Was just using heat expansion to give me water flow thru the system. I used a DC fan to blow thru the radiator to move some heat around...
This was back in the days where you always brought a big bag full of D-CELLS along with you haha...
With todays toys this hydronic water heater layout for small trailers would work like gang busters in a tent setup...

Roy Ken