First - Welcome to the forum!
I would base your inverter size on what you want to run off of it. If its only the fridge you can probably get by with 1500-2000 watts. There is generally not much of a price difference as you go a little bigger and you might decide to add a few plugs off the inverter at a later time. Would definitely use a pure sine wave inverter.
Zack and Donna plus Millie and Ranger
2018 Montana 3160RL
"Life is too short to stay indoors, enjoy the ride!"