Go for it, however, some rules of etiquette:
1. If he's at a shop or regular stop, get on the truck or wait outside till everyone is off. If you do get on and its packed, get off & wait.
2. Don't interrupt him if he's with a customer, wait patiently, then introduce yourself, tell the man what you want and set up any arrangements.
3. Make sure you discuss terms first. Most dealers(including myself) will not finance someone off the street, unless you can provide ID, credit card, business info. etc. It may be best to start small, paying in full, until he gets to know you and feels comfortable with giving you credit.
4. If the man is eating, give him a break, we don't get much time, if any, for lunch. You could ask where his next stop is and meet him there.
5. Don't be an *******. I don't know why but I've had 2 different guys get on the truck for the first time and both turned into complete a-holes. It's a fast way to booted.