I would think the brake shoe is 1/16"and the lining is 3/16" when new & bonded vs. riveted. There has been a recent thread on here about brake dust & wear. Three years is not a lot of time unless your brakes are dragging or wearing prematurely. You might talk to the rv shop as to their experience.
In my case, I purchased the trailer in 2003 and have not changed the brake linings yet. It took quite a while until I got them adjusted to where they were same braking force. (when my wife winces in the passenger seat & causes me to lock a wheel, then I know which one is adjusted too tight)
My linings are not wearing much. Two trips to Texas, one to Florida, Internationals, British Columbia caravan, Newfoundland caravan, plus misc. trips. 20,000 plus miles.
Are you going to Alaska on the caravan with Makovich????