Currently I'm using a small solar panel because my aux battery doesn't tie into my truck's alternator or any type of charging system. The previous owner put it on a small trickle charger to charge it up before he went out. I like not having any connections between the truck and camper so if I need the truck bed, I can just remove the camper and not worry about disconnecting this or that.
At a local car dealership I noticed many of the cars had a small solar panel on the dash with a plug going into the cigarette lighter. When I asked about it, they said they use them for keeping the vehicles charged up since sometimes several cars sit there for days and weeks without being turned over. The salesmen got tired of trying to show a vehicle only to find it had a dead battery, so they use these small (14" x 6") panles.
What's great about these small panels is they also have battery clips in addition to the 120v plug in. So I bought one from their parts dept and keep it in the camper. They also come with 4 suction cups, so I can stick it to the large window on the pass side, on the inside, and clip it to my battery to charge while I roll. I can also set it out on the back bumper in the late afternoon to charge the battery while I make a fire and dinner.
I like it being portable as opposed to mounting it it one place. I can move it where the sun is instead of trying to park the truck where the panel will get the most direct light. Another benefit is, should I kill the battery in the truck by leaving a dome light on or something, I can use this to charge my battery back up. At least enough to get the engine to turn over. Might take a few hours (I have no idea, haven't needed to do it yet) but it sure beats hiking out and paying AAA to come jump start me on a mountain!
I'll try to get some pics and a product name of mine put up later. For the space and cost I think solar panels are a great option, especially if you boondock a lot.