What chassis is your coach built on . Gas/diesel ? Air brakes or hydraulic?
Do the rear lights on your coach use the same bulb for signals and brake lights?
If you have a common bulb for signal and brakes then here's some testing.
If you have power through the brake light switch with the brakes applied the next item in the circuit is the turn signal switch . You should have power to one of the wires to the signal switch, from the brake switch , at the base of the steering column ( a connector with 7 or 8 wires) there will also be a power feed from the hazard flasher and when the ignition is on the signal flasher.
With the brake pedal depressed there should be the power from the brake switch to the signal switch AND two other wires , the ones that go to the rear of the coach. If you have power up the column to the signal switch from the brake light switch and none back down , you have bad internal contacts in the switch and need to replace it.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.