Replace the inlet with a hose connection ...Perko type. Then run the fill hose inside . You can put the tank where you like and not cut up the outside skin. This is your Airstream......make it yours. If your in it for resale just do the work first class....no trashy work. Don't paint the inside with a cheap latex....You won't hurt the resale value. Do it cheap and half way.....your wasting your time.
"If it can't be reduced, reused, repaired, rebuilt, refurbished, refinished, resold, recycled or composted
then it should be restricted, redesigned or removed from production."