This weekend I changed out my stock Marine/RV Duty Group 24 to 2 6V golf cart (Costco) batteries. I also hung a 12V inverter near my television so we can watch movies without having the generator running (we rarely have hookups).
I had to modify the battery area a little bit to fit both of the 6V batteries, but it only involved a few extra pieces of hardware and a short length of 2x8. The inverter is held in place with industrial grade velcro.