I've asked two caravan gas guys , one said yes it's fine , and one said no its not , but neither one could really give me an answer why.
I think I've just stumbled across the right answer , by asking Google a slightly different question . It seems that decent heaters(ones with oxygen depletion cut out)are all made to a standard for using lpg , and either propane or butane will be fine . Just keep propane bottles outside , don't sleep with the heater on , and use two reliable carbon monoxide alarms.
I might still be fitting a diesel night heater instead , it's just the solar panels an