Two 6-volt golf cart batteries!
Easiest and cheapest.
You said no solar. That is the wrong answer. Reconsider. Unless you will drive from outlet to outlet and always be either plugged in or driving (alternator power) then solar will make life so much easier. Solar itself is really simple.
The problem with Marine battery is that it is neither made for starting or for RVs. It is made for a boat. To start the boat motor, go somewhere, turn off motor, then use electric trolling motor for some fishing, then start the motor again and return home. It neither is a good starting battery or a good RV battery, but a compromise, and really only good for a boat.
True deep cycle batts like a GC batt is much more robust and will last longer than a compromised starting/marine battery.
