The car/tractor dealership used it on lot for snow removal.
Welcome to OTT dot_bob.
So it's not a brand new tractor. Can't tell from your profile how much snow your area might have received or how old the tractor is so the amount of use or abuse is a question.
Snow and mud tracked in was probably never cleaned until it came time to sell it causing the mold you speak of. They may have just taken a hose to the interior to flush the dirt out before selling it and let the water sit in puddles. Clean it up as best you can and make sure it's dry.
Check door seals for cracks or tears and proper fit when the doors are closed. Make sure the "new" guy told to go clear snow didn't damage them somehow.
Check roof for holes from lights or mirrors that might have been removed. Seal or reseal any holes.
Once all clean and dry try the hose as rudder35 suggested. Have someone spray while you're inside or visa versa.