89 E250 Transmission identification

I just bought an 89 E250 with the 5.8 EFI V8. This is a coachman campervan. The transmission is not shifting vary well between 1st and 2nd. It will constantly shift back and forth when in a low speed area. Also I do hear it shift to third but once up to speed, 55 – 65 MPH the engine seems to be running a bit fast. I am a complete laymen when it comes to automatic transmissions, this is the first I have ever owned so I tried looking it up and I find they had both a 3 and 4 speed available and I am now wondering if it’s the 4 speed and it’s not shifting up from 3rd. I assume I have a vacuum issue regardless and I want to change out the Modulator Valve but I need to know what valve to go get. Can anyone help me ID this tranny?

Ok after doing some more research I am convinced it’s a C6 transmission and that means three speeds. I still would like to replace the modulator but when I try to look up the part I get a bunch of choices such as Purple line, black line…….I have no idea what I actually need.