My Cherokee is on LPG (propane) and I noticed a whiff of gas at the back end today. It's got two 60lit tanks in the back which are vented to the outside. I found that one of the safety valves had started weeping. Legally they have to be swapped every two or three years, mine haven't been looked at in quite a long time. You are not allowed to repair them yourself even though it's only a change of a rubber washer, they have to be calibrated to a specific pressure. I'll have to get two new ones at around €37 each ($39)