This subject is also about space, that is getting the biggest amount of total power possible.
Not knowing every RV model, but guessing now they provide a larger space than a standard 12 v car battery. I have what looks like space for 2 regular sized car batteries but installed are 2 6v batteries.
As stated before do not parallel batteries, it is not safe to do that without isolators.
So work from your dimensions and find the biggest batteries that will fit. There are differences in amount of power for the same size. Basically the more they cost the higher power (more lead plate).
Math is math - Total voltage (adding batteries together) times the amp hours of (one battery) when in series gives you the watts (work).
Now another piece of battery 101
Batteries will deliver more total power when discharged slowly. The difference is big.
Also never use a battery below 75% as now you start to cause life limiting damage. Some 12 volt equipment will not operate at low voltages.
Last never leave a battery mostly discharged, charge it right away.
So for you inverter people to do it right you need LOTS of power to avoid deep discharging the battery at a rapid rate.
My suggestion is to have a 24v inverter with charger and 4 - 6v batteries just to operate the inverter.