Possible ground issue?

Is this the shop that worked on the APU?

If not, then they need to contact that shop and ask what they did.

It seems that this happened after they worked on it, not before so the point that started all of this was the APU repair.

I am thinking that there are several possibilities with this and it sounds like a lack of a ground happening, like one on the APU generator (the thing does not have to be running when the truck is running) or they recabled or left a cable off the batteries or if they messed with the alternator, the ground strap is missing (many of them need it even if it is "mounted" to the engine with four bolts).