That is a battery in the box. And if you were to do enough research, you would likely learn it is not used by Forest River coaches. To check the electrolyte levels, disconnect both battery cables, then removed the battery from the tray. Once the battery is on the ground, the electrolyte levels can be inspected and topped off if needed. This auxiliary battery is supplied by GM as part of the cutaway RV cab and chassis. I have redone the wiring of my coach to use the GM auxiliary battery as it was not used for anything on my coach. If having no interest in using it. I recommend disconnecting the auxiliary battery lead from the auxiliary battery relay on the inner right fender and removing the battery from under the coach.
Go here to learn more about that battery: