as opposed to small popup camper rims of 8" that spin twice as fast as a 15"
YES you should!
From Dexter axle web site:
BEARINGS - How often should I grease the bearings?
Along with bearing adjustment, proper lubrication is essential to the current function and reliability of your trailer axle. Bearings should be lubricated every 12 months or 12,000 miles.
BEARINGS - What is the proper bearing adjustment procedure and recommended grease?
Please review the Dexter Service Manual for instructions and specifications relating to bearing adjustment and lubrication. WARNING: It is important NOT to mix different types of grease thickeners. The grease that Dexter Axle uses has a Lithium Complex thickener. Mixing our grease with a Barium, Calcium, Clay, or Polyurea soap based thickener agent will cause adverse affects. This may include causing the two greases to harden, separate, become acidic, or pose other hazards and damage to the bearings.
Good idea to check the bearings once a year. We annually take our TT south to the Gulf and travel/camp on the outer banks a lot. Sugar sand can, and will find a way to infiltrate every seam and seal. Will eat up anything that rotates. Goes for your TV too.