..Every TT and 5er I've ever owned stated to level unit first THEN slides out...Don't know about MH, never owned one
As far as using slide supports, we camped close to a 5er that must have rented site for the summer?? not sure, but they had been there for awhile. Don't know how long 5er had been there without anyone checking on it, but it had all three slides out with supports underneath the slides and one side had two tires that were very low on air...slide was being pushed upward about 3-4"....No one around to tell, but wondered if it damaged the owners slide out, it looked bad. Most manufacturers don't recommend using them under one's slide for the very reason I mentioned above..