I just finished installing a 120AH slimline AGM deep cycle battery with a 20A DC to DC charger with MPPT solar regulator. I ran a cable from the main battery, through the firewall and under the door sills to the back, so it charges the battery from the alternator when driving and the 200w solar blanket when camping. Plenty of power for a fridge and lighting, etc. As the battery would have overlapped the picnic table (by 10mm), I've built a bit of a deck/shelf 10cm high, so I can still access it and the spare wheel well I use for storage. Built it out of MDF and pine over and around the wheel arch. Just thought it might be of some help and inspiration if anyone's thinking of doing something similar. The slimline battery was a great find and the way to go.