For replenishing the on-board tank, I switch the two-way winterizing valve so that the pump draws from the antifreeze inlet and just connect a hose to the inlet and stick the other end in a portable container. I then open a valve that I installed that sends water from the pressurized side of the system back down the suction hose from the tank
Another way I use this valve is when I fill the tank I can hook up to the city water inlet and just open the valve. That way I don't have to stand there and hold the hose in the gravity water fill opening (I do take the cap off for better venting while filling tank).
I am considering getting one of those water bladders (has anyone used one?) and just run the hose from the antifreeze inlet to the back of the vehicle where the bladder is at. Then I wouldn't have to ever lift a water container in or out of a vehicle and there would be no big jugs to store somewhere.
I am thinking about adding a valve under the bathroom sink that would do the same thing (move water from pressure side back to tank), but draw from the hot water line. That way you can get hot water all the way to the bathroom sink, dumping the cold that was filling the pipe back into the tank
The upshot is that if you are cold weather camping and worried that nighttime temps could get a bit low you would put a bunch of hot water in the storage tank to give it a bit of thermal flywheel for the night.
I have also thought about adding a sprayer for the toilet, fed by a pump that pulls water from the gray tank. This would use gray water to flush/clean the bowel. The gray tank fills WAY faster than the black anyway so you get two birds with one stone.
I try to do my part by hydrating from alternative sources, but DW and DDs/DS can't help much with that