Manufacturer of panel shouldn't matter, just match voltage as mentioned. I'd also match monocrystalline vs polycrystalline. It most likely doesn't matter, but I'd still match anyway. Mono should be more efficient. Here's an article on it:
Panels are roughly $0.80-$1.00 a watt, so a 100W panel will be $80-$100. Cheaper are fine. The higher cost is usually from marketing or name brands.
The Wynns did a good video on series vs parallel connection and how/why to use one or both.
how shade effects power
I'd end up with a mix of both if you go with 4 panels. Just need to make sure the charger Jayco uses supports 24V. The higher voltage will also be good to decrease power loss and better utilize the higher gauge (thinner wires) Jayco uses is the solar panel prep.
All the work should be on the roof of the unit. So 3 panels at max $300 (assuming 100W panels), a few connectors, maybe $20-30 and then labor. Shouldn't take more than 2-3 hours at most since we're just installing panels and everything else is already wired up and running from factory.