My coach has the same problem, but at the opposite end! The filler is situated so you can't get even a quart bottle to work without making a mess, and it takes 19 quarts to fill. So here is what I made. The one end just fits inside the filler tube, and the other end is just big enough for a gallon jug spout to fit.
Maybe you can fix something up using PVC pipe that would work for you?
Good Luck, Be Safe and Above All, Don't Forget To Have Fun
2006 Fleetwood Discovery 35H, 2014 Honda CR-V, M&G Engineering Braking System