Best way is to leave the trailer plugged in to shore power - doesn't work for you.
Second best is solar - doesn't work for you, unless you consider somehow getting a panel temporarily fixed atop your cover, on the ground, etc. Probably would get stolen or damaged, so I guess you don't want to do that.
Plugging into truck - takes far too long and you'll be sitting around for days.
Plugging into generator - again, takes way too long, although it would be faster than your pickup truck.
Depending on how long your storage period will be, what might work best for you is to just pull the negative cable off the battery bank. Or install a switch to do the same thing, but make sure it kills everything. Most OEM switches are not wired to totally isolate the battery bank. When totally isolated, fully charged batteries will last for several winter months. But you do need to make sure they are charged before you put the rig to bed, which usually will happen as you travel home.