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

Either that or just go with a couple batteries and kill them in 30 minutes or so.

No way can you run your ac without having a huge bank of batteries, way more weight than your RV can carry.

You could go with Lithium Iron phosphate batteries that you can discharge to nothing without harm but the cost is prohibitive, about 800 bucks a pop. 4 of them would cost more than your older RV is probably worth.