This is some thing that may help.
I used to smaller 550 cca batteries and the original battery tray reworked so i had plenty of room for both batteries.
I think both batteries sit length ways in the compartment instead of cross ways of the original battery.
This gave plenty of room left over to fit any other electrical stuff in the battery box.
My vehicle is a TDI 300 Defender 1994.
I used two simple battery oscillation switches as they are 12 dollars each and take up very little room.
( the marine items most people use is expensive and is big and bulky.)
The dual switches have another advantage is the plastic keys are easily removed and act as a vehicle immobilizer .
Most fridges these days have a low voltage cutout protecting the battery from damage these days.....deep cycle or not.
I find the 550cca standard starting batteries are cheap and do the job.
One will run a modern compressor fridge for two days in normal conditions and the vehicle will start OK on one normally charged battery ....no problem.
I have noted some people manage to get two 750cca batteries into the defender battery box. but it is very, very tight.
Another mod needed is a 3mm wire by passing the battery switches and running up to the dash board to run the radio , alarm system and clock or you find you will be resetting these items every time you turn off both batteries.
I generally run both batteries together and just isolate one If running a fridge and camping lights over night etc.
jayco2005campervanhawk26122007028.jpg picture by 101Ron - Photobucket
