How do I install my new fresh water tank?

if the butchered hole is to large to be covered by the new gravity fill get piece of alum sheet and cut a plate large enough to cover all the damaged area then cut the opening for the new fill in the plate and secure both to the trailer with screws or pop rivets but be sure to seal between each piece, you don't want a leak into the wall. The alum plate can be painted to match or polished. if that doesn't appeal to you then the next option is to purchase a new section of siding.
Once the fill is installed you will connect with plastic food grade hose available at most big box stores and Ace hardware. the hose most likely won't bend that sharp with out kinking so you may need a pvc 90 degree elbow to place in the hose.

Another option here, although maybe not as desirable, may be to use a vent cover to cover the butcher-job on the aluminum, and then run your inlet through the end wall, straight to the tank. You could use a gravity fill or a filler like Vikx shows here, either one, then the barbed fittings and plastic tubing to connect.

I agree with Number 10. An elbow from the "hole" to the tank won't allow for easy filling. Unless you can move the tank forward enough to relax the bend in the fill...

I'd check out fills at the RV shop and figure out what you want. There are two different barb/hose sizes (1 1/4" and 1 1/2") but they are NOT PLUMBING FITTINGS. In other words, you must get the fittings at the RV store. Try to use all the same size fittings and hose. (match the tank) Reducers are impossible to find.

Once you have the parts, place the fill above the tank. Don't forget you will need a vent tube as well. Some modern fills come with a handy little tube connection for the vent. Otherwise, you can vent thru the floor.