Well, I assume it must be somewhere in the Rocky Mountains. Since some movies are already set on a real life location, I made some research on the other locations that are made up. Maybe this could help you.
Live Action Movies:
Scooby-Doo(2002)- Moreton Island, Australia
Scooby-Doo 2(2004)- Vancouver, Canada
Mystery Begins(2009)- Vancouver, Canada (Templeton Secondary School)
Curse Of The Lake Monster(2010)- Santa Clarita, California
DTV-Movies:
I am going to skip the WNSD-Movies, since they are mostly set on real life locations.
Abracadabra-Doo(2010)- Maybe Montana, USA
Camp Scare(2010)- Somewhere between Colorado, Idaho and Utah- USA
Legend of The Phantosaur(2011)- New Mexico, USA
Music of The Vampire(2012)- Somewhere near the Atchafalaya River, Louisiana, USA
Shaggy's Showdown(2017)- Arizona, USA
Happy Halloween(2020)- California, USA (Since I believe Crystal Cove is based on some city in California).
This is just my opinion. I skipped a lot of movies, since I didn't know on which location they were based of.