Dash Air

On a post 1997 F53 chassis, check for a vacuum canister located in the lower right front corner from underneath. There's a short vacuum hose that connects a 'Y' fitting from the engine to the canister that's subject to dry rot. I use a short piece of rubber fuel hose to replace it that has a much longer life.

Dutch_12078 wrote:
Which chassis/engine do you have?

The OP says he has a gasser Baystar. According to the Newmar website, the Baystar has

Newmar wrote:
Every Bay Star is built on a next-generation Ford® F-Series Class A Motorhome Chassis with standard Electronic Stability Control. It features a 7.3-liter V8 with 350 horsepower and 468 lb./ft. of torque, plus new heavier duty sway bars for an even ride.

Which chassis/engine do you have?

When that happened to my Winnebago Class C, it was the famous vacuum leak. Squirrel chewed vacuum hoses, to be precise. When you looked down into the engine, you could see a squirrel nest. I had to have all of the vacuum lines under the hood replaced.

I then took a 12v tail light bulb and socket and put battery connections on it. I placed it under the hood so the engine compartment was always lit up when parked at home, and left the hood cracked open so the critters would not feel secure in there. I kept the MH plugged in 24/7/365 with a Trik-L-Start sending power to the engine battery, which kept the engine battery from running down from the constant light bulb.

Depending on the rig . . . If it is a diesel Like Winnebago there is a vacuum pump up front in the generator compartment. If it is a gasser it is tapped off the intake. Either way you have a leak or a bad vac pump if it is a diesel MH. The vacuum hose usually cracks and that is the culprit.

You have a vacuum leak or disconnected vacuum line.
The system is controlled by Vacuum "motors" and if vacuum is lost DEFROST being the most important function is the default.

On a car the vacuum is normally tapped off the brake booster but on a Motor home the brake booster is often hydro, not vacuum. so it's tapped off the intake manifold. However the leak, break or disconnect can be in many places including the selector valve or the motors themselves.