I have not hesitated to ask a farmer if I could pay a few bucks to fill up my FW tank from his hose, assuming the water was potable. However, I live in Texas, so things might be different.
I have also bought a bunch of five gallon water containers, put them as best as I could between the rear axles of my TT on the floor of my camper, which luckily was where the dinette is, so I just shoved them underneath that on a rubber mat, and they stayed put.
I'm not sure what your rig is, but for me, additional water isn't too tough. I just toss containers into my truck's bed or in my rig. When moving to a motorhome, I'm considering an open-frame trailer as another place to put additional things like water containers, fuel cans, and so on.