I have three smaller AGM's in one bank, located driver's side under kitchen counter top cabinet just in front of the axles. I have another bank of 2 X 6V FLA's located on the passenger's side in a vented battery box I built, also just in front of the axle wheel well. I wanted the batteries inside to keep tongue weight down and to afford a good temperature when camping in cool weather. It also offers some security above the typical tongue mounted box.
Vented box:
AGM bank:
AGM's under this shelf--
Behind a cabinet door (the one with gauges)--