It sounds like you have done everything, but just to be clear:
1. Connect cable to outside jack
2. Turn Booster off
3. Set source on TV to the same you use for antenna. There is no source for cable, it uses the same input on the TV as the antenna.
4. Scan for digital/analog cable channels - there are at least two types of cable systems, standard and HRC. Most are standard.
If the above is what you have done and it doesn't work, your trailer may have been modified like mine. The PO put an additional splitter in the wall behind the plate where the switch is and used the outside cable line to drive another TV set outside. You will have to pull the plate to see if yours has been modified. Check your antenna manual or download one and compare the wiring to what you find. There is a small circuit board behind the plate. It has two inputs, antenna and cable, and one output in addition to the one on the plate. If you don't have a coax connected to the cable input, that's a clue. Mine had nothing on the cable input, an extra splitter connected to the TV2 output, and the wire to the outside cable jack was connected to the splitter.
"You cannot reason someone out of a position they have not been reasoned into"
Al, K5TAN and Missy, N4RGO
2002 Classic 30 Slideout
2001 Safari 25 RB Twin (Gone, but not forgotten)
2013 Dodge 2500 Laramie 4x4 Megacab Cummins
WBCCI 1322, TAC FL-39, AIR 82265