I had a 1989 E350 van camper that would,after making a long hard climb up a hill, start smelling like there was a gas leak. I'd stop and check but couldn't find any problem. When I went to remove the gas cap, I'd get a real strong pressurized blow back of air/gas mix. After that, things seemed okay for a long time until a long hill climb or a long travel day when it was hot outside and then the problem reoccured. Problem was the oxygen sensors were faulty and dumping a lot of gas into the exhaust system which plugged up the catalytic converter which turned beet red and the gas tank and the rest of the rv was super hot as a result. Changed the O2 sensors, catalytic converter and things settled down.