(it should be noted that I wrote this before reading KMP44's response. I don't disagree of with any of his assertions including the bulkiness of moving and storing solar panels!)
Shade is definitely a problem with solar but you don't have to have direct sunlight to generate electricity. You will get some charging in the shade. I think having solar will direct your choice in campsites going forward! You will be looking for those nice bright sunny spots!
The problem with portable is you have to find a sunny place in that already shady spot and move the panels around. And how long does that spot stay in the sun. You don't want to spend the day moving a panel from place to place. And finally getting sufficient wire size to support proper voltage to the charge controller is an issue especially as the length goes up. Heavier wire, longer length harder to store, drag around, etc. We camp in shady spots too. The sun is directly overhead in all of them at least for a while. And in the summer maybe enough.
As for batteries: I have two Crown 6V. They are equivalent and perhaps better than the Trojan T-105 that I had in my previous rig. Both are great and that's the way I would go. I have not worried about having a spare as I've never had a problem with properly maintained batteries. I suppose if push really came to shove you could use your tow vehicle battery. There is an discussion about two twelves vs. two sixes elsewhere in this forum. Until somebody brought up battery failure in that thread I had not considered it and certainly have not worried about it. Having a third, spare, battery aboard would give you one more thing to worry about: is the spare charged? Is it being maintained properly? Might as well take care of the two. Again, I have never heard of a battery (especially a relatively new one) failing. Could happen I guess. Worst case go home!
Yes, roofprint is usually the place to start in the design: make sure you can put some more or less standard sized panels up there to begin with. Most rigs have stuff mostly down the centerline leaving space out on the edges. If you look at Mud's rig you can see what I mean. In my case I have my panels forward on the rig with the vent fan for the bed between and then for the other two panels the AC is between them. There will be some shading of the panels by the AC cowling but it won't last long and I have 600W up there which is probably 300W more than I actually need. Back to the oversize thing. Take a look at some possible panels and get rough dimensions: usually 20-30" wide by 45" or so long (width is more important than length generally). When you get your rig head up top (don't worry about walking on the roof) and measure everything. Since you've already scoped out the panels you will know where you can lay them on the roof. You can also see where you will be able to fish wire down to the charge controller and batteries. Lots of ways to do that. I have not seen too many roofs where I could not put the required number of panels. The 100W panels I looked at on Amazon are 22" x 45". Lots of places to put those on most all rigs. The 150W panel is 27" x 58". Remember you can safely mix and match (again, the arguments will rage and technically everything said will be accurate but none of it will matter in this scenario).
I'm a real practical guy. I want to see as many people as possible use solar. I don't want to listen to generators when I'm camping! Ever.
Take a look at my blog shown below (if you haven't) to see photos of several rigs and how the panels were laid out. It ain't rocket science!
Show Low and Rio Verde, AZ