Bruce's link is golden.
I think you're spending too much for what you're getting on that 18 w. from Camping World. I have a 30 watt, and its the minimum I'd recommend. It keep my one group 27 charged when I'm diligent about keeping it deployed all day, but there isn't much to spare, and I don't use much battery to begin with.
I'd look at one in the 30-50 watt range, more if you want certain full charge all the time, or want to recharge in shorter time. Lots of info already posted about portability. I just put mine inside during transit, and set it on the ground leaning against the trailer during use, or set it on the roof if there's no chance of wind and the sun is high.
Knowing what I know now, if I were to need to buy one tomorrow, I'd get a 50 or 60 watt.