No rooftop a/c's on a 2006 Horizon. Basement air.
Have the sealants checked on the roof around all vents, TV and radio antenna, etc. Also check the front/rear cap seams on the coach, clearance lights, any protrusion thru the roof and front caps. Then the seams along the roof edge where the awning rail tucks in, both sides.
There is probably a leak from one of these areas from the roof, which is migrating to where you're seeing the spot on the roof.
Once it's sealed, the area should dry out, and the ceiling material can be cleaned to remove the stain.
First order -- find the leak and fix it.
'07 Winnebago Journey 34H - CAT C7, Koni's, MCU's, SS Bell Crank, Safe-T-Plus
'07 HHR Toad, SMI AFO, Blue OX