Dash AC blows out of the defrost vents

You should make sure there is a check valve installed between the vacuum reservoir and the port it is connected to on the engine. This will hold vacuum in the system when engine vacuum is low. You can purchase one at any parts store.

The vacuum line that goes to the vacuum servo on the HVAC box under the hood and controls the outside/inside air door at the passenger footwell is prone to cracks and pinholes. It is easily replaced. You definitely have a vacuum leak somewhere. Good luck.

The Ford V10 engine does indeed supply vacuum for the HVAC controls as equipped by Ford. On long WOT hills though, as Big Katuna mentioned, the vacuum can drop low enough that the door actuator switches to the fail safe defrost position. As also said, adding an aftermarket electric vacuum can overcome the drop in engine vacuum. The one I added is an aftermarket replacement for the pump Ford uses in their diesel pickups. If there's a coach manufacturer that added a similar pump at build time, I'm not aware of it. I have seen various owner and dealer installed pumps though.

The 2000 F53 has to have a vacuum pump. There is no vacuum provided by the engine.
You have a 12 volt vacuum pump with a vacuum canister, under the hood.
Start there to find your leak

It’s a common problem as there is very little vacuum at WOT like when going uphill.
Many have installed a small vacuum pump that runs when vacuum gets low.

X2 on the vacuum leak that Krusty described.

We had a similar thing happen to our Class C last year, except that the A/C was coming out of the defrost all the time. In the Class C the vacuum reservoir is buried and very hard to get to. I wound up buying a new reservoir, check valve some vacuum tubing and fittings and cobbled together a bypass. I never found the original leak location but all is well now.

Krusty wrote:
Yes, its a vacuum leak. Have a look under the hood for a vacuum reservoir. Test it for leakage. You will need a vacuum pump for testing. Also have a look at any vacuum lines for cracking etc. You may have a vacuum check valve near the reservoir. Test it as well, when they get worn, they don't hold vacuum well.

thanks, will do.

Yes, its a vacuum leak. Have a look under the hood for a vacuum reservoir. Test it for leakage. You will need a vacuum pump for testing. Also have a look at any vacuum lines for cracking etc. You may have a vacuum check valve near the reservoir. Test it as well, when they get worn, they don't hold vacuum well.

Cant speak to that specific model, but many vehicles in the last 20 years have had computer systems aboard that when certain conditions exist, causing any problems with vacuum or heat control systems, the computer defaults to only service the Defroster with any climate control air.
My Buick did this when the evap coils had a hole poked in it.
That it returns to normal likely shows it to be a control item designed into the system.

You have diagnosed it correctly.....mostly.

Mine did that occasionally just because of a lack of vacuum when pulling hard without downshifting. It got much WORSE when I loaded a "custom tune" because it tended to stay in the top gear longer.

There might be a vacuum leak but it might just be "normal" too.
Adding a larger vacuum tank or an aux. vacuum pump can completely fix the problem.
Not using the cruise control in hilly conditons can help too.

P.S. ALL of the RV shops I went to just came up with a blank stare.
I finally found a truck shop that knew what they were doing.