Keep in mind the Fifth Wheel storage area is under your bed in most cases so I would want to have any battery installed in that area using a vented box... You can use any kind of box you wish but you will need to tie the vest hose into the existing vent hose the other battery box is using to vent the bad fumes to the outside..
This GOOGLE VIEW link is many different type of battery boxes available with vent hoses...
PPL and Dyers comes to mind... Just click on the GOOGLE IMAGE of the one that catches your eye you might want to install. Just make sure the box will fit your GP SIZE battery you are going to use.. You could make up a pretty good battery box with vent and hose parts from LOWES.
VENTED BATTERY BOXESRoy Ken