How to reload/refuel maintenance trailers?

I've driven through the repair zone/service hub near the garage 14 times with the towed yellow maintenance trailer hooked to the back of my truck on Michigan Dam map, it neither repairs anything, refills anything, nor refuel anything.

All over the internet I see people saying to simply drive through the zone & your vehicle will be repaired, your gas tank refilled, and the maintenance trailer attached will be restocked and refueled. What am I doing wrong?