Hi and welcome to the forum. With either a 5th wheel or a Travel Trailer, the usual method for leveling side to side is to install a bubble level on a forward facing part of the rig. Pull into your site and get out and check your bubble level. If the bubble is high on one side or the other, use a block of wood, usually at least 2X8 in thickness and approx 12-15 inches long. You will need one under each tire on the low side of the rig.
Place the block on the ground, next to each tire (say on the left side if that side is low). Get in the truck and drive forward a few feet. Get out and slide the wood blocks sideways in the tire marks where your rig was just parked. Try to be precise as you can, you want to have as much of the tire on the block as possible. Get in and back the rig straight backwards slowly until the tires are fully resting on the wood blocks that you just put down.
Now check the bubble level again. If the bubble is close to the center of the level, you are done leveling side to side. Then unhook the rig and and level fore and aft as usual. For an out of level site you will always level side to side first. If the site is way out of level you may need to stack another block on top of the first one and repeat the process.
It gets easier every time you do it, and eventually you will be able to tell how many blocks you need by where the bubble is on the level when you first arrive. The bubble levels are available at any camping store like Camping World, most any RV lot that sells towables, or even at Wal Mart. The wood blocks are often scraps of left over wood from a building project, or just buy yourself a length of 2X8 or 2X10 and cut them into desired lengths. Camping stores also sell plastic leveling blocks that stack on each other for a bit more money.
Your filling up water tanks sounds like the "tank fill" lever is in the wrong position. It is probably in the water bay, and is probably labeled as "City" and "Tank fill" or something similar.
Good luck!