I assume that you are using hydraulic levelers. If so it is a little surprising as those levelers act on the coach's frame and should not tilt when more side load is put on them with the slide out.
But in any case I would just tilt the coach a little against the slide when you level so that it is all level when the slide is out. That assumes you can control side to side leveling.
How much is it out of level when the slide is out starting with a level coach?
David
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