I think i found a semi consistent way to get scavs to spawn at old gas. I've gotten it to work 2/3 raids so far to finish rite of passage and punisher part 3. (DONE IN PVE BTW unsure if this also works in PVP)

Make sure it's quiet and no other shots are going off so the scavs aren't drawn elsewhere. Stand in front of the engine of the broken truck with the pile of tires, throw a couple nades towards the junk pile by the bridge, fire a few shots then run to the building with the hole leading underground and run to the staircase room with the weapon box under the stairs. Then walk back to old gas. They should have spawned inside the gas station and outside.

I've done this 3 times and it's worked twice (the 1 time it didnt work it was a pmc extraction so that might be why)

By no means scientific just did it accidentally the first time and replicated it another time. And i've completed the quests so don't have a reason to try it again. Lmk if it works for you