Saul, your post confuses me. You say, "on the same side of the door". The "same side as what" - what else is on the same side of the door? What do you mean "the end of the trailer lower". Are you saying the trailer lower half? Which end, the front end or the rear end? Finally, a "valve" is kind of like a "switch", but it's usually found in a pipe to control flow of liquid or gas. I have no "valves" on the outside of my trailer, other than the one at the lower curbside corner that's used to empty the fresh water tank. Is that possibly what you are asking about?
Are you possibly talking about the filler port for fresh water that's found on the front of the lower half of the trailer above the battery compartment? This port is behind a small door that can be locked. If so, you just put a drinking water safe hose in this port and add water until it starts to come out past the hose, indicating the tank is full. Then, you use the water pump inside the trailer to pressurize the lines and get water from the tank to flow to faucets and the toilet.
Hi-Lo 1707T - Tire Minder TPMS on Tow Vehicle and Trailer, 300W Solar Battery Charger, Equal-i-zer WDH
2014 F150 Platinum 4x4 3.5L EcoBoost SCrew