Ham license for 35 years, GMRS license for about 8 years. I have a CB in the Class C motorhome, just AM no SSB. Antenna is a 2 ft Super Flex mounted in the driver's side front hood channel with a fender bolt mount, and tuned with an antenna analyzer. Resonates on 27.275 MHz or so, but it doesn't cover the whole band with better than 2:1 SWR. The 2 ft short length is needed to fit underneath the cab overhang.
The setup is good enough for road conditions on Ch. 19 or vehicle-to-vehicle chat when in a caravan. I've done some range testing with the pickup truck which has a 9 ft whip on the rear, and the range of the 2 ft antenna CB setup in the Class C motorhome is somewhere between 1.5 to 2.5 miles in gently rolling terrain. I'll put the radio in 40 channel scan mode in a campground from time to time, but there's very little CB chatter in the campgrounds anymore. GMRS/FRS is of more utility when camping.
Channel 13 is the RVers channel. Been that way since the CB magazine "S9" was publishing suggested channel usage charts in the 1970s. See page 66 of the October 1979 issue of S9 for example:
https://worldradiohistory.com/Archiv...S9-1979-10.pdf