Finally buying a strong inverter. How many batteries to buy?

You are getting in over your head, both financially, what you expect to be able to run and at least as critically, I doubt your 1984 Chevrolet 27' class C could stand the extra weight of an extra-large battery bank. Also, your alternator was not designed for charging deeply discharged large battery banks.

Save you money and get a generator large enough to power what you want to be able to run.