My experience with the Lance campers that I have owned (835, 981 & now 880) has been that the battery compartment is just barely able to accommodate the size that it was designed for. I remember specifically with the 981 wanting to switch to 6v when I replaced a couple of Interstate Marine group 27's. Unfortunately the 6v were taller than the 12s and they would not fit in the compartment.
My current 880 has one group 27 which needs to be inserted at an angle to get it in. I went with a maintenance geptreeted because I knew that checking fluid level would be a giant pain.