Battery or batteries

If you never camp without hook ups one battery is fine. We dry camp often and can go 3-4 nights watching a movie every night on our TV running through a power converter. We use twin group 27 deep cycles bought at Costco. All depends on your style of camping. A cut off switch is great for several reasons. I use a simple push pull switch mounted atlo the battery box.

The fuse should be sized to protect the wires from the batteries to the 12V distribution panel.

A battery master disconnect switch should be installed in every RV, IMO. Most newer RV's have parasitic draw that is drawing down the battery 24/7. Those devices are things like alarms, circuit boards, radio and TV memory/standby, etc. It's a good idea to wire the emergency brake power lead bypassing the switch. If you forget to select the "on" position before hitting the road, the E-brakes will still work.

Go with two batteries
Use at least a 30 Amp circuit breaker instead of a fuse. You can get the circuit breaker at auto parts stores.
A cutoff switch is a good idea. When storing the trailer the cutoff will prevent parasitic draw from some appliances to run down your batteries.