Hi , I'm not familure with your chassis, but in 99% of vehicle wiring, the signal from the bake light switch goes to the turn signal switch, where the power is split to left and right then it goes to the rear of the unit to power the brake lights. When you put on the signals the switch interupts brake light power and substutes flasher power. On a gas MH the power from the brake light switch can be tapped into at the signal switch wiring at the base of the steering column. You will have to test all the wire to the signal switch to see w itch is the power from the B/L switch and not an output to the rear lights. Unless you can find a wiring diagram for the signal switch. If you have a diesel pusher, it gets a little more complicated as the engine brake may activate the brake lights also, causing the trailer brakes to come on.
If yours is a D/P, re-post and I'll walk you through that system. Chassis type would help in any request for assistance, F/L, Spartan ?
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.