You can actually fit a 140 amp hour battery in the standard battery tray.
I use one like this
12v 140AH DEEP CYCLE/MARINE/4WD AGM BATTERY 2yr warr. (eBay item 290351422012 end time 12-Jan-11 14:59:13 AEDST) : Cars, Bikes, Boats
with 1350 CCA and 140 amp hours.
It should be enough to run the fridge and camp lights without the additional expense of a second battery, battery tray, battery charging system etc.
It worked for me because my old battery was on its last legs, whether its worth replacing a good battery I dont know.
It require a little modification, I cut out the bottom of the battery tray,
put thin rubber over the sharp bits that may abrade the battery.
I also wrapped the battery in a sheet of strong tarp I had.
The posts cant be fitted, so flat type battery starter cables connect the battery to the leads (supercheap auto, or repco)
The top wont fit back on, and its too high to clamp down, but it is a VERY tight fit as you can imagine, and I cant see it moving even in a serious prang.
The top doesnt fit back on, so its exposed in all its glory.
If you like I can post some pics, its one solution.