Thanks for the info. I already have a cell water kit installed. That and a cutoff switch was one of the first additions after ownership and maybe a reason I have had good service from it so far.
The chassis battery is still operating OK, but the fact that it is a Harris and that it is going on 4 or more years old, I though it would be wise to change it now by choice rather than on a trip at an inconvenient time.
Removing the folding desktop, pan and plywood plug was very easy with a screw gun and razor knife. I installed a wood cleat on each side of the opening to mount the plug back and close up the hole. Now, to go shopping.
I have a few pics I can post if you think they will help, although there have been some good ones posted already by others who have HAD to change their chassis battery.
Oh yeah, I found a couple of wires that needed some tie wraps, so while in there, take your time and look around for loose wires if you go in.