I was told many years ago by the service rep to jack under the u-bolts and have been using that method for as many years with 4 different campers. I also use a 20 ton bottle jack which I carry in the truck for both. Like the height of this jack.
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John, Jan & Buddy
2012 Chevy 2500HD LTZ Duramax CC 4X4
2013 335SS Cruiser Patriot