I've done it. I own a 1988 turbo diesel.
First, choose a deep cycle battery, not a starting battery.
I chose a 85 Ah/20 hours battery
You put it the compartment behind the seat, either driver's or passenger's.
I had a problem, the battery is 3mm too high, I was not able to close the cover.
I modified the hinges and put a 4mm spacer...plus an insolating sheetunder the cover, just in case.
From the electric side, I used a salvaged copper jumper cable, connected to the main battery through a relay controlled by the fuel valve wire. The two batteries are interconnected only when the engine is running.
Be sure to protect the line on both end with 32 Amp fuses (You have two batteries, two sources of short circuit)
A refinement worth the money: an undervoltage protection between the aux battery and your camping electric network.
This prevents overdischarging and ruining the battery.
I use this system from the beginning, more than 20 years. I changed the battery every ten years.
My 2 cents