1: YES!
2: You want at least 100 watts for the effort. It will be charging most of the day depending on use and size of the battery. For really being off grid you need to be 300+ watts with a larger RV.
3: Depends on the roof material. Most just screw a bracket to the roof and seal as appropriate. Your roof has lots of holes already for vents etc so it is nothing new.
4: Most run down the fridge vent, tank vent, or simply direct through the roof under a sealed junction box.
5: If you camp off grid or store the RV without power then yes it is worth it.
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