This job does require a modicum level of skill.
I assume your 13 ft has no trailer brakes
. Even so, you don't want to be pumping so much grease in the bearings that it oozes out the inner seals.
That only makes a mess, and is not good for the tires
Disassembly, cleaning, and repacking with clean grease is best.
It will require removal of the inner seal which is likely to be damaged, so a new seal is always recommended.
Seating the bearings to proper clearance is critical.
Do you have a level floor or driveway? Do you have a good jack, and know how to use it? Tools? Can you handle the weight
of a wheel and tire?
Once you have the bearings properly cleaned and greased; you can check if they are running OK by feeling the hubs after a long run on the highway. Cool to slightly warm? OK, Hot? Not good.