It would seem very hard to carry every spare part that might break. In 40 years of towing a trailer I have never had to change a wheel bearing on the side of the road. Only one tire in that time frame. I would think regular inspections of the wheel bearings and other related parts to be an easier approach. JMHO
Jeff, you are correct, to some extent. I inspect brake assemblies and wheel components annually but that doesn't guarantee problem free. We are full-timers so we are in our rig most of the time. I carry some spare parts, mostly those that, if one should fail, I can get to a safe parking area where I can either get some help or repair the problem myself. This is one of those parts and they don't take up much room. When we first started full-timing, 7 years ago, we attended the National rally in Gillet, WY. Glen, from Dexter, was at that rally and gave a most informative presentation on maintain the entire axel assembly. He recommended carrying the extra set of bearings after relating a story about an RV'r who had a problem with the wheel assembly and there was a need to replace a bearing. After his roadside provider arrived, he would have been able to complete the repair but, by the time he arrived and diagnosed the problem, and determined he didn't have the correct set of bearings with him, all the part stores were closed. So, they were forced to leave the RV there, luckily at a rest stop, overnight. Long dissertation, short morale, cheap insurance and more peace of mind. Your choice.