2007 Georgetown 340TS w/ same issue
Just an FYI for everyone as I had the same issue, with my 12V/lighter outlet, Map Lights and Radio going out just as I was de-winterizing my rig . I bought a 12v tester and dug in thinking that the same issue with field mice that had affected a few other people at our storage facility. Turns out (thanks to the forums advice) it was actually a blown 20A fuse in the fuse block on the firewall in the streetside corner. I removed the cover, and started pulling the fuses one-by-one until I found one that was blown. Flipping another 20A fuse into the vacated spot and the radio and lights came right back on.
WHile I was under there, I added a second pair of 12v outlets that I put under the dash, next to the driver window, so I could place my GPS and mobile in a better position instead of in the only spot on the dash console (which I always hit with my knees...).
Hope this helps other folks out!