We have considered Pemex stations in Tepic more than once but never actually stayed in one. We did ask and they have always been fine with the idea - no problem - plenty of room - park wherever you want. I found two on Google Earth....
21.527520 -104.929868
21.520377 -104.926110
The first coordinate - two Pemex stations across the highway from each other - obvious trucks park there.
Second coordinate - one station on the southbound / eastbound side - I think this could be the one where I asked.
I thought there was one close to the Xalisco/Campostela turnoff but I don't see it now. We once stayed at a Pemex near Zapopan with no problem - exhaustion had set in and darkness falling made it sort of an emergency stop but it turned out just fine.
We don't normally stay at a Pemex if we can find an RV park of some kind but there are times when there is no option. We have never been refused or had any kind of problem staying at any Pemex.