This adventure appears to retain a faint echo of the show's original concept, that Shaggy and Scooby are musicians. They appear to be going to the music festival as performers.
The mention of Interstate 5 implies that the story takes place on the West Coast of the United States.
This episode is undoubtedly influenced by The Ghost Breakers (1940.) In both movie and TV episode, a pointing portrait is the clue to the direction of a treasure. Both also contain a clue referring to "Marching Men." Both settings are also a creepy, decaying, supposedly haunted mansion.