Manifold/header/muffler/tail Pipe problems of the 2000 Mitsubishi Montero Sport

Truck has had a smell similar to natural gas in the cab, especially when idling. The first time we brought it in they checked and found no smell. Then we brought in again and found that the camshaft seals were leaking and replaced hoping this would help the problem. The smell consists and we now have trouble when driving and the transmission is slipping in 3rd/4th gear. Both the dealer and Mitsubishi feel that the smell is from the type of gas and is normal. We have switched from mobil to shell to montori and it happens with them all. This a severe exhaust like smell and we are afraid that it is not healthy for our small children, and it should not be normal to deal with.