elevationlvr

Hello i am a first timer here, thanks for having me. I have a 97 f-250 heavy duty, 460, 4x4, jet chip, the exhaust is cut at the muffler (rear axle). My truck has this fuel / varnish smeel that is very strong into the cab. Doesn't matter if there is a load on it or not. seems to be in 1500ft elevation and up, or when it gets 100 degrees or more in town. A month ago we pulled the 5th wheel (7000lbs) up about 3500ft elevation in 1 1/2hr ride. Had to pull over because the smell. I seen alot of fumes or vapors comming from the charcol box right under the batt. I went again for 10-15 min, no smell. Then it comes back again until i pull over and let it rest for 10-15 min. Before i pulled my trailer home, i took it to a shop and was recomended they do a full tune up. My fuel milage got worse? it was 8.2 now is 7.4??? Please help. Thank you.