I know this is an old thread, but I have to bring this up; Most of the info I got about the GMC RTS variants were from this book;

https://www.amazon.com/Bus-World-Encyclopedia-Buses/dp/0961983000

And I agree with the OP. They're not called "Mark 01," "Mark 02," "Mark 03," "Mark 04," or whatever.

Some were delivered in late 1980, but they all had "B" (1981) for the date code in the VIN.

That's not a bad collection of information. One thing though: GMC RTS-04s were introduced as 1980 models.

Clarification on the RTS series - 01s were built in 1977-78, 03s from 1978-80, and 04s from 1981 (some deliveries were in late 1980, but as 1981 models) to 1985. 06 production started in 1986, which was the only year for GMC RTS-06es. There was also one 05 (T-drive) built in 1984.

RTS-06 production continued under TMC (1987-1994) and NovaBus (1994-2002) alongside two new series: 07 (T-drive), of which only three were built by TMC in 1992 and 1994, and the more popular 08 (wide front door) built by TMC from 1989 to 1994 and NovaBus from 1997 to 2002. The front modules of these look nothing like the 01/03/04/06 and even went through changes throughout its existence:

1989-92 TMC RTS-08
http://gallery.bustalk.info/displayimage.php?pos=-8648

1993-94 TMC RTS-08
http://gallery.bustalk.info/displayimage.php?pos=-5619

1997-2002 NovaBus RTS-08
http://gallery.bustalk.info/displayimage.php?pos=-15260

The TMC RTS-07 front clip was similar to the second iteration of the 08:

https://www.flickr.com/photos/organistmike/2369087179/

And finally, a 2006 Millennium Transit RTS Legend, which is a T-drive but narrow front door, unlike the wide front door 07. I believe they are still considered series 07.

http://en.wikipedia.org/wiki/Millennium_Transit_Services#mediaviewer/File:Njtmill- ennium4101c8nx.jpg

I'll just change how the site displays the number to have 04 instead of Mk4 then

To make clearer: RTS buses are classified by certain destinations. There is 01 (1977), 03 (1978-1980), 04 (1980-1986), and 06 (1987-2002). Snookie's "Mark" thing could have based on this, but can still be considered incorrect. I suggest not using "Series" to ease confusion and just either go for the number or not at all. There are also some RTS'es misidentified based on old info. I will drop notes about the incorrect info on the incorrect buses. Thanks.

I just thought that I should make a note: If you saw all of the comments I have made on "RTS" buses, I just want to say that "Mark IV" or "Series VI" is incorrect. They should be just be "GMC RTS" or "TMC RTS" or "Novabus RTS. I just wanted to drop that note so it can be changed. Thanks.