i just bought a new truck and there is a second battery tray. My camper's battery house is quite small and i would like to run a second battery in parallel. i think this is a fairly common application...
I never had a good luck to deep cycle/marine batteries. They were coming with 2 years warranty and always die a month past the warranty. Cheaper car batteries come with 100 months warranty. The only difference is that you SHOULD not discharge car batteries too low, but I had one that was shorted overnight (headlights) and discharge deeply couple more times, yet lasted longer than the warranty.