FOUND IT. On page 286 of pages 8512 in the 1999-2003 Service Manual on disk, it says
"The air conditioning (A/C) fan pressure switch is located on top of the receiver/dryer. The switch is normally open, but closes as a result of rising pressure. When the switch closes, the fan relay is energized. The switch requires 1620.3 kPa (235 psi) +/- 206.8 kPa (30 psi) to close."
There is no mention of A/C systems in the 2004-2005 service manual on the same disk.
2005 30' Rexhall Vision, W-22 chassis, 19.5 tires, 208'' factory WB, 6000 CCC