I replaced the house battery in my 2007 Interstate with an Exide Orbital battery after a cell in the original went bad. There was very little room under the passenger seat to wiggle the old out or the new in. However, there is a way to accomplish this task.
The passenger seat rides on two runners when you adjust the seat for and aft. At the far forward end of the runner track are tabs that prevent the seat from moving too far forward and sliding off the track. If you remove the tabs (hack saw) you'll be able to slide the seat forward enough to remove the battery easily provided you raise the seat using the two lifters on the right side of the seat. Be careful not to let the seat rail run off the track. There are small delrin bearings that may come out.
When you're through with the battery installation, you can insert screws into the tracks where the tabs were to act as a slider stop to prevent the seat from coming too far forward.
I haven't actually measured the space under the seat but it looks like there might be room for two 6volt house batteries in series to give a higher capacity.
After replacing my battery, I think I have a problem with the Parallax charger. It doesn't charge the battery. I'm checking that out next week.
Has anybody had an issue with the power converter?