FWIW, I get about 3 years out of any of my batteries. 2 6V for the coach and a 12V for the starter.. Brand doesn't seem to matter....so I go for the bucks. Sams' or WallyWorld. (Series 27 12V and GC 6V).
What you can do with the 12V (suggestion) is get blade switch and put that on either terminal. Disconnects the battery. It might extend the life of the battery, but sometimes when you reconnect it, the computer has to re-boot, causing what is called "driveability problems" until it "finds itself". Really not a biggie, but be aware of it.
As far as the car/dolly question, if you have not already purchased this car, do more research and find one you CAN tow 4 down. A dolly can be a PITA, but to each their own.