1997 Chevy Astro AC stops blowing out of dash vents at high

You have a vacuum leak under the dash. The vent doors are actuated by engine vacuum. The added load of the trailer forces the engine to work harder. Thus, intake vacuum is lowered. If there is a slight leak, the lowered intake vacuum and the leak combined allows the air flow to change directions.

Check under the dash. If you are lucky, you will hear a hissing sound with the engine running. If no sound, you have to check visually. Look for cracked, dry rotted lines.

Let me know what you find.

Joe
PLEASE NOTE: Check under the hood for the same type of problem if nothing is found under the dash