1986 Ford Econoline E350 Motorhome

My husband and I own a 1986 Ford E350 Coachman motor home with a 460 engine. It has 27,000 original miles. This year when we unwinterized it and I took it on it's first trip, it started to backfire and lose power cliimbing hills. Previous to this, it was a real runner, with no problems climbing hills. My husband is retired truck mechnic (18 wheelers). He figured that water was in the lines and I put in two containers of dry gas. This seem to solve the problem at that time. He then decided to get into it and replaced spark plugs, air filters, gias liine filter and some hoses. I again took it on another trip and it seemed to run fine. After filling up again this weekend and it started to back fire and lose power uphill. I barely got it up hills to a safe place to pull off. I called him and he said to use octane booster and dry gas again. I did that and it ran ok, some backfiring and some loss of power, but nothing as bad as it was before I used those products. IHe is stumped and is hoping for any other ideas as to what it could be. Also, the engine runs fine, no high idle. This engine also has double belt system. ( My husband is not here to write this, his grandmother died and he is out of state with his family, so I am trying to remember all that he told me, he is not internet savy at all.)
Thanx, maagk