Poke around under 'Tow Vehicles/Towing' or do a search on U bolts. There's a lot of discussion on this.
You'll most likely find that you have to do this multiple times when your rig is new. It takes time and miles to settle in the bolts. I check mine every 1000 miles or so and always get some movement. My last check was around 5000 miles and I only had two that were down maybe 5 lbs or so. That was my 'best' check ever.
Be sure to check your axle manual for the correct spec on torque. It varies depending on U-bolt diameter. My axles are supposed to be 45-65 and a Dexter rep told me he goes to 75 on mine. It's a matter of some dispute I guess, like what's the best truck (Dodge)
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