Someone got them in and hooked them up, so it is possible, just not easy. It is going to involve some sliding around and probably tilting through the opening. It might take two people strong enough to handle the batteries at arm's length. I can barely handle some of the larger sizes close to my body.
I suggest putting plastic caps over the terminal posts before trying to get them through the opening as it would be real easy to short both posts on some part of the RV. The caps are on most batteries when bought new, and you might find caps also in an auto parts store.
