Propane furnace blows out

Thanks for the feedback. Yes the furnace runs fine when parked, never had an issue with it shutting down. As far as I can see up the exhaust tube there are no nests and I have a screen over the tube.The furnace is located a little further than half way down the passenger side and it has always failed to run while on the road. It is only an issue for a day or so when we run down from Canada in the winter.
I am not sure what the propane pressure is and will have to arrange to have it checked if this persists.