Since its a single cylinder and probably not going to be run 24/7, I doubt there is going to be that much excess heat that you can effectively capture to heat a tank of water in your camper.
Regardless of practicality, I was thinking of a
PC water cooler thermocoupled pump might be adapted. However, from just looking at MEP-831a pictures, I don't see any hot flat surface area that you could mount it on that would give a solid heat transfer.
On the other hand, perhaps you could remove the muffler and tightly coil wrap some copper tubing around the engine exhaust outlet/inlet pipe to muffler and braze it in place to obtain some heat transfer. Whether its going to be enough to heat a tank of water via a heat exchanger in your camper's hot water tank is doubtful though.