The reason Border Protection want this is so they can check up on whether you are really there, or going there. (It's virtually neverrarely used, of course). Putting the address of a Walmart won't help that.
My recommendation is to put the address of the place you are renting the RV from. Then, when you get to the border officer, tell them your plans and tell them why you put that address and ask if that's what they wanted. It's 99.9% likely that will be fine, and the worst case is that they ask you to fill in the form again.