Rv Trailer Brake and Gear Selector Issue

Okay so I have a 2003 ducthmen class c motorhome with a ford e450 chassis. A trailer brake box was installed at the dealer when I bought it. Long story is that I have had no issues till recently. I went to put the gear selector out of park and with foot on the brake the car would not let me move it. So, I hit the trailer brake box to manually turn on the trailer brakes and with foot on brake pedal I can move the gear selector. The other strange thing is that I have no emergency flashers and no brake lights when I hit the brake unless I am manually holding in the trailer brake box. So,I bought a new brake box and installed it exactly like the old one was but still the same issue. I also bought a new brake pedal switch that goes on the brake pedal and still the same issue. Lastly, I checked the blue 15 amp fuse under the hood and it was fine!! At this point is there a fuse somehwere causing an issue. Maybe the battery could be a little low? I am just confused and looking for any help cause the vehicle and trailer brakes worked for years until now!! Thank you