It used to be (might still be) common practice for truckers to mount small CB antennas on side view mirrors.
Clamped to the top of the ladder on back of motorhome is another idea. Run the feedline under the motorhome along the frame and up one side of the ladder,
Since the walls of the motorhome are fiberglass, an antenna mounted on the bumper should work fine.
Here's an example of one that mounts on a window. The RF energy is transferred through the glass.
Tram 1198 Glass Mount CB with Weather-Band Mobile Antenna - Walmart.com
You can get it in combo with a CB/Weather radio for $77.40
That should get you started.