How about just picking up a portable jumpstarter?

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=40615

It's portable, 24Ah, already comes self contained with the necessary outlets, and no need to mess around with complicated charging systems, just charge it from your car cigarette lighter. Plus it comes with cables so you can jumpstart your car if your main battery dies. I have one wired up to one of my vehicles right now, so it's always topped off when the car is running. I was going to go the dual battery route also, but I did this in the meantime and it works well enough that I might just keep it like this.

I think a solar system would be more trouble than it's worth. Even if you were to get one large enough to put out decent power, it wouldn't be portable anymore, and the smaller ones aren't always capable of recharging SLA batteries. A car alternator can recharge the battery in a fraction of the time that it would take the solar panel. Carry an extra 1 gallon gas tank and you have enough fuel to recharge your batteries for a really long time.

Edit: I see that swampdonkey already brought up the idea.