StingrayL82 - An option is to have the gas full inlet moved to the top of the tank.
This will be difficult because most welders won't go near a used gas tank, oil drum, etc., due to the risk of a fire or explosion. I'd look for the absolute best welder in your area. (If I was in your shoes, I'd be willing to travel 400 miles to find a reliable welder willing to do the job.)
I do both oxy-acetylene and wire feed welding. (I was taught 'no filler' welding and a friend was amazed when I showed it to him.) I wouldn't touch a used gas tank even though I know the procedure for doing so safely.
Even if the tank is complete emptied and purged, enough fumes remain to create a danger. In addition to completely draining and purging the tank, safe procedure involves filling the tank with water. Creating an airtight weld is very tricky due to the water drawing heat away from the weld. The tank also needs to be well vented because the water will boil, creating vapor pressure inside the tank.
If you choose this route, consider carefully where to place the new tank inlet. You will probably need to modify your coach to reroute the fill hose and possible move the exterior fill fixture.
You should also plan on resealing the tank with epoxy fuel tank sealant.
(I really hate thinking of something I should have added to a post after I hit the send button.)
