This is a key point and will affect the adoption rate by trucking companies.... along with charging network.
It seems really really likely though Tesla will have a quick response mobile service team set up that can deal with things quickly. At least for the short term while there isn’t a huge volume of semis on the road. Later, when there are substantial numbers, it will get tougher. Inevitably it will help that the semi has substantially fewer moving parts than diesel trucks.
Tesla does have a lot of experience with mobile service it can put into play here early though.
Likely there will be more problems with the trucks early, then these will over time get ironed out and eventually semi will be notably more reliable than ICE trucks.