Black walnut issue

I think the size of the fabric you need means big wind loads, even if the fabric isn't a solid surface. Those fabric roofs at Home Depot have steel or aluminum frames anchored to the ground to hold the fabric in place. You're going to have a much harder time engineering a solution that hangs from trees with some bungies and/or chains and a strong wind could end up pulling some of the branches down along with the shade.

Can you put a cover on the RV? If you still want to use it occasionally, some covers have zippers on the passenger side to let the cover be easily rolled back to gain temporary access to the RV.