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X3 - After having the same TT for a few years now I know the level and know exactly where I want the bubble after the "pre-slide" leveling so that it comes out perfect when I extend the slide out.
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Exactly what I do. With my trailer, I know that the slide will tilt the trailer a tad to one side when out (and I do mean a tad). This means, pre-slide, my bubble is a little off center, but...when I put slide out it centers perfectly.
Slides will vary by weight, hence- when you go to know your trailer, you'll know where the "pre-slide" bubble needs to be on the leveler.