Could be a number of things of course, but are you SURE your ground is good? Sometimes this can be deceiving.
I had a similar problem with the jack on our 2005 Classic 30. The ground seemed good when checked with a meter, and indeed the light worked - but the jack was dead.
I suddenly noticed however that when I switched the jack on, the light would go out (and the jack wouldn't work).
In checking more closely, I found that a wirenut connection in a ground wire was full of corrosion and the wire inside was almost completely rotted through.
So the ground connection was there - and good enough to run the light, but could not pass enough current to run the motor.
I just cut the wire at the wire nut, stripped back to clean copper and soldered the two ends & insulated with shrink tube.
Jack runs great again!
Of course I have no idea if this is the same problem you have ..... but maybe!
Good luck ............. Brian
Brian & Connie Mitchell
2005 Classic 30'
Hensley Arrow / Centramatics
2008 GMC Sierra SLT 2500HD,4x4,Crew Cab, Diesel, Leer cap.