I have installed a second enclosed box in our CrossRoads. I drilled a hole in the "floor" for the vent for the box bottom. Both halves seem to be the same. I purchased a pvc "T", 2 pvc 90 degree elbows and a 2' section of pvc pipe. (I do not remember the size) Measure the vent on the box or take it with you to the home improvement store. The 90s will fit over the vents loosely. Use pipe to connect 90s to T. add a short piece of pipe to T "outlet" and attach existing vent hose to that. I hope you get the idea. I added a removable "pin" holding a 90 to a box vent. I dont think it is needed but...
Installed the box using self tapping screws. Had to do some cutting on the boxes to allow the cables in and out. Creative cable routing will make connecting and disconnecting easy. About the only drawback is lifting the batteries in and out of the boxes.
When is storage, the batts are removed and the floor vents are filled to prevent critters from getting in.
You can find the boxes at other places. Google "RV vented battery box". You won't need the hose/vent kit