Hey Ghost - good luck on the search. Guess Rd is off my beaten path but I do come through Bethesda (Hwy 70/Sherron Rd/Miami Blvd) once in a while.
There is a North State Storage on Sherron Rd, just north of Hwy 70 (at the intersection of Miami, 70 and Sherron). Might be a couple miles more convenient than Brassfield, if Brassfield is the one I'm thinking of off Page Rd south of 70. I'm pretty sure I can remember seeing boats & RVs out there. Don't know anything about security/cost/covered/uncovered, but might be worth a call if you haven't found them yet.
A Public Storage facility shows up on google at N Duke St near Holt School Rd.