I'll add some things not said or maybe not realized.
Yes, torque the lug nuts. OK that has been said... And yes a trained mechanic who has been torquing a certain size fastener for years has a good handle on the feel of that size fastener. However not everyone falls into that group. A torque wrench evens out the tightness and is a procedure others can follow and come out with the same result.
Many trailers use 1/2-20 studs. They are generally grade 8 carbon steel studs. If you look up most bolts charts, a 1/2" fastener tops out at 120 ft. lb. dry on grade 8 carbon steel. Notice I said dry and it tops out. That torque lines up with 75% of the yield strength of the fastener on a grade 8 bolt. And that means it is tight enough to have a pre-stretch in the bolt to not fatigue over time from a loose fastener, yet the fastener can be used over and over and have a high reliability of not breaking or stretching. The 90 to 120 ft lb range is a common recommendation on the 1/2" wheel stud. The reason they stop at 75% of yield is as once you approach yield, permanent deformation sets in (the bolt stretched and does not come back) Running past yield the fastener will fail in time. Reaching the tensile limit and the bolt fails totally regardless of time. Those not using a torque wrench do not know where they are in this % of yield group if they have not been per-calibrated some how. 80% of yield will not get ya, however the higher you go the worse it gets and then there is bolt manufacturing tolerances.
Lube was mentioned, lube changes the friction and thus increases the tension with less torque. Depending on the lube you use, you need to use less tongue to accomplish the same 75% of yield. A common de-rating is 20 to 25% of dry threads torque. Meaning if you use a say, teflon based anti seize and the spec was 100 ft lb dry, then 20% to 25% less is 75 to 80 ft lb. Each type of lube can have a different K factor which is how the lube changes the friction. If you want to read up on this more, here is a reference we use at work often from Fastenal (a big name in the industrial fastener supply world) start on pdf pg 24.
http://www.fastenal.com/content/documents/FastenalTechnicalReferenceGuide.pdfNew steel wheels can be the worst some times. Why? Most camper wheels are painted with a nice heavy thick paint. And that paint ends up jammed between the nut and the wheel. Over time the paints works itself out and the fastener can be loose. Once the paint is gone, then at least you do not have that to deal with. I agree and follow the 50, 100 and 150 to 200 mile guide to torquing after any wheel change. The 1st 50 seems to be the worst and by the 3rd time at the 150 to 200 mile mark they have settle down and do not move much more. That said, under any heavy breaking, it is wise to recheck and before long trips and at a periodic time that fits your towing habits.
It is hard to give a one size fits all about twice a year or once a year as some do not track mileage. Some tow 3,000 miles a year, some 10,000 miles a year. The per year does not really fit in this case. But they have to give something that the average RV'er can follow and is a good guideline. Even at the start of every trip does not fit all. What is meant by the start of a trip? If you are doing a 1 month towing trip of 5,000 miles or more, by all means do it before and during the trip. If a trip is 3 times a year and you are going 50 miles to camp, well once a year fits as then you have to remember what year you did it... So there is the catch all of, do it often.
If you have manual adjusting brakes as many campers do, it is recommended to adjust every 3,000 miles and that fits. Do a torque check then too.
Hope this helps and happy camping
John
