TIMKENs recommendation is roughly .004-.008 Clearance on a Tapered Bearing. Someone mentioned The harbor freight style hand held Infrared PYROMETER, I recommend you own one for checking bearing and tire temperatures as a preventative measure. (HOT TIRES are a sign of overload or under inflation. Never exceed Factory recommendations from vehicle or tire manufacturer, for air or weight.
Always better to have a loose bearing then a tight one. I have over 80,000 miles on a Alpenlite 29RK 5th wheel. I changed a few bearings with new Chinese ones...what a mistake that was....they came apart in one year...roughly 12,000 miles. I have two 6,000lb axles 8 lug nut Lippert axles.
on the scales both trailer axles weigh 9900 lbs dry and 6700 on the drive axle of the truck, which is a 2003 dodge 1 ton 2WD. Which is really right at the GVWR for the truck drive axle tires. 3400lbs each.
When I replaced the bearings the second time, I hunted on ebay for AMERICAN MADE TIMKEN....OLD STOCK from individuals. It took some time, but was able to get Genuine USA races and Bearings. I think Dexter says..pack bearings every year or 12,000 miles...which seems a bit much for me as a retired heavy equipment mechanic. But at least if your going on the road for many months...2-3...or more. You have comfort of knowing you are in good shape, hate the fear factor.
Timken has many videos on you tube.