I have a 40W solar panel on the roof, with a solar charge controller, connected to a yellow top optima, which runs my engel and aux lighting when parked up for long periods.
I got it on offer from Maplins for £106, and I thought at that price it's a worth an experiment (
http://mobile.maplin.co.uk/module.aspx?moduleno=510795 ). At 14.7kg it's a little on the heavy side, and measuring 1253x643x37 it takes up a bit of space on the roof, but every mod is a compromise.
I haven't done any specific testing to measure the full benefit, but whenever I've stuck a meter on the battery you can see it's charging the battery, and the charge controller has LEDs on it to indicate when the battery is either charged, charging, or low.
Due to it being an 'amorphous' weatherproof solar panel, it seems to charge the battery from dusk till dawn. I have seen the charging led on before 5am in the summer, and late into the evening.
I do know that when parked up on baking hot summer days, the engel would drag the yellow top down to 8.6V after 2.5 days. This year I was parked up for 3.5 days and the fridge was still happily running away. So yes it does make the aux battery last longer, but how long it'll last till it can't run the fridge depends on so many variables, I couldn't give an exact figure.
Variables are battery capacity, solar panel size/efficiency, size/fridge current draw, is the fridge in a thermal jacket, are the car windows tinted or reflected, how hot is the air temp etc........
I very happy with mine, and at the price I thought it's worth ago. It's been on the roof for 5 months and has survived some encounters with tree branches, and some vibrations from offroad driving at speed. If I was buying again, I would look at the new ultra thin solar panels that are appearing on the market, but the price of them may not be so favourable, yet.
Hope that's of some help.
Cheers.