Solar panels are the easy, quiet version of powering your RV off-grid. As long as you have sun and a decent setup, you are fully self-sustaining.
Mounting solar panels on your roof means more upfront work, but less work over time. You can also continue charginging your batteries while driving. With portable panels, you'll have to bring them outside and set them up when you want to charge, but have the advantage that you can put them where they'll get the most sun.
The wattage is how much power the panels generate. The higher the wattage, the more power (and larger dimensions) - typically you'll see 100W, 160W and 175W panels, and most people buy several to get their desired amount
Monocrystalline panels are a little more space-efficient and work a bit better in low-lighting, but are also more expensive. Since you're likely limited on space either to store or mount your panels, Mono panels tend to be better