I just did exactly what you were thinking. I got a 4 GC2 battery box, mounted it under our bed and installed 4 6V AGM Batteries. The batteries weren't cheap! I spent about $1000 on batteries and didn't even get a "Cadillac" brand. Hopefully they will last us a good long time though. I did have to route some 4GA welding cable through to the converter -- went down through the floor, over to side, along to rear of trailer and back up. About a 15 foot run.
AGM was the only way I would consider mounting batteries indoors, especially when the logistics of my situation prevented a "high" vent to get some convection going through there. Also my underbed box was just in front of the axle so it helped the weight distribution of our trailer.
