Inverter Help

[Rwork]

I have a 2021 3781RL Montana Legacy and it is missing the inverter from the factory. Factory basically said, "too bad." So, I wouldn't recommend a Keystone product to anyone but doesn't help me now. What I need is your help/recommendations for installing an inverter.

What size and brand do you recommend, I'm thinking I'll only need one large enough to run the residential fridge while running down the road?
With or without an auto transfer switch?
What size batteries?
What brand and size of cutoff switch and fuse link?

Thanks for your help! At least the Montana Owners group is the best even though Montana as a company doesn't have much integrity.