A balanced flue takes in the combustion air from outside the vehicle and exhaust outside the vehicle.Also. What do you mean "balanced flue"..... It's my lack of knowledge in these areas that scares me ?
An unbalanced flue (if that is the term) takes it combustion gas from inside the vehicle burns it and send the exhaust outside.
This may not sound an important difference. But if your flue gets partially clogged combustion gases can end up inside the vehicle. If there is a back fire which does happen in LPG appliances then it can shoot flames and combustion gases inside the vehicle.