If you are going to do it yourself, you can probably get by for $20K or less. Get a quote from Mueller Steel Buildings for one of their prefab kits. I built a 30'x40' metal building on some land I own 12 yrs ago using one of those. Built it for a tractor and other implements, not for an RV. Had help putting up the frame and the roof panels. Rest I did alone, including insulation. Pretty easy, but time consuming. Kit was just over $10K and concrete slab about $6.5K with extra aprons. That included extras such as 2' overhang all around, steeper pitch, gutters, insulation, extra windows and doors. Would be more of course now.
If you intend to have it built by a contractor, expect $35-40K or more.
I would definitely go with a 14' high x at least 12' wide or more door as some 5ers are 13' tall or more and you want plenty of side room for maneuvering in/out.
If I were to do it again, I would go with conventional garage type doors rather than the standard corrugated roll up type doors. They do not seal well at all and cannot be insulated. They just ride in tracks up each side. High winds blow dirt/leaves in around the sides/bottoms.