Trailer wheel balancing question.

"Never had them balanced." Is it only a problem when you can feel it in the truck? By running unbalanced tires over a long period do you suppose that could have been part of the cracked centers, loose lugs, etc. that you referred to?

No....for several reasons.
Your incorrectly assuming because the tires on a trailer weren't balanced on a machine they cause vibrations on the trailer.
The biggest reason tires on a loaded trailer (rv or non rv) don't show out of balance wear patterns like a truck tire is those short 26" long very stiff non progressive leaf springs on a trailer vs a 70-76" long soft riding progressive leaf springs on our trucks.
Lugs on trailers become loose more from side scrub stresses.

Cracked centers are not caused by a out of balanced tires on a trailer....but rather side scrub forces from backing or sliding around corners on close spaced axles on our trailers.

All trailers can have shaking just from rolling down the road due to road imperfections/rr track crossings/etc causing Loose screws/open doors/loose molding/popped fasteners/etc and all can happen with balanced tires according to those that have their tires balanced and had those issues.

As has been brought up many times before in trailer tire balance discussions is that trailer hubs are not machined balanced as automotive hubs on our tow vehicles.

If someone has a tire jumping up and down on a loaded or unloaded flatbed trailer or a RV trailer then its time to get the defective tire off the trailer