Seems odd they would have been doing any work inside where the 12V
wiring passes but who knows. Adding an easy start should all have been done from the outside of the trailer. Maybe they were messing with wiring to the AC controller and impacted the 12V
Check the 12V
thermal breakers / fuses first.
Next ask the dealer just what they touched. Were they messing with the in trailer wiring?
If you can't get a straight answer or if you do and they were messing with the wiring, you can access much of it by removing the Return air filters, and poking around up there. most of the 12V ceiling wire goes through that area.