Scooby-Doo! And The Loch Ness Monster
Chillman came to Drumnadrochit in his van because he had heard about the new evidence of the reality of the Loch Ness Monster: the damage it did around the Blake Castle and the footprints it left. That damage was done by a land-going robot operated by the Haggart bros., hired by Prof. Fiona Penbrooke.
Later, Penbrooke hired the same two brothers to steal Chillman's van and disguise it as a second land-going version of the Loch Ness Monster. She also had a water-going version (a disguised submarine).
All of this was done to convince Sir Ian Locksley that Nessie was real so he would take her photos seriously. The brothers did it more as a prank than for the money.