When you are close to a trailer with fuel the refuel UI appear.
The same function is unavailable for trailer carrying repair.
Repair function unavailable(trailer) PTS 6.3
It's been like that since initial release as far as I can recall. I've never quite understood that concept of having to hook up to a service trailer for repairs when you could drive up to any sized fuel or maintenance trailer and fill up.
Because of that model, I will always send a couple dedicated trucks out just so I can "switch over" and do a repair. Otherwise what good is having a maintenance trailer in the field?
There is no need for a yellow zone! You can refuel just by being in proximity of a fuel trailer. The same should apply to Service Trailers!