There's a whole lot of difference in bearings in your tug than that of your trailer. The tugs bearings are carrying people. Consideration for safety, factories used bearings that are over rated for the load. Most of the bearings are sealed for life these days. The tugs bearings are larger diameter and with larger diameter tires
the bearings are turning slower inducing less wear. The tugs bearings are not sitting around for months at end collecting moisture. Moisture in the bearings causes early bearing failures. The smaller the bearings and trailer tire diameter and the longer the trailer sits unused, the more frequently you should service the trailer bearings. I feel that the yearly bearing service comes from boat trailers and that they use smaller type tires
and they dunk warm bearings into the water. Its a hard life for those bearings.
Far easier to service a set of bearings at home before a trip than on the road in the middle of nowhere.