We carry 2 - 6 gallon jugs from Wal-Mart also. A carry over from our Pop Up a few years back.
Unfortunately, the jug nozzles alone are of no use with our TT so I made a portable filling funnel, adapted to fit into the FWT opening. Got it from Wal-Mart as well. I took off the flexible hose and replaced it with a 3' rubber water hose. Cut the opening of the funnel tip a bit larger to snugly adapt the hose through the hole, to pull through, and long enough to go down into the FW tank. This bypasses any air pockets that I have and works perfectly.
My new TT needs some assistance when filling the FWT due to back flushing. This mod fits the bill perfectly when having to fill with jugs, when away from home. We are good!
This is the funnel I used to improvise my water filling solution.