ive been looking for a heater with out a 20min cut off but cant find one any where in fact I don't think they do one some thing to bear in mind or am I wrong
20 min built in safety cut out is there for a reason. Chepo lpg/propane portable water heaters without one fitted or disabled have been known to catch fire/explode. Not ideal when yer van goes up with it

Seriously though to be honest I've never found the 20 min cut out any issue myself as turning the flow of water at the wfp end on/off whilst working will re-set it. Another way is to use say a one shot wirless remote that you can turn the pump controller off/on whilst working that's if it's in range.