Depends on what you are talking about. There are Winnebago parts and there are Dodge Van chassis parts.
For the Winnebago coach portion, appliance stuff is mostly from RV supply houses. Trim and body parts are going to mostly be from a junkyard. Winnebago simply has very few of the parts still available. A few that are still available are listed in this catalog (large pdf file): http://www.winnebagoind.com/resource...go_Catalog.pdf
I hope that you did not purchase this Class A Dodge parts manual for your Class C minnie: 1969-77 DODGE MOTOR HOME Chassis Only, Text & Illustration Parts Book
. It is only for Class A, not Class C.
I am not sure what the Dodge MB300 van chassis parts looks like. If you do have one of those then you look up the Dodge P/N and then have the auto supply person cross reference that number to the after market P/N. Just be clear on the fact you have a MB300 chassis, not a B300 (passenger van) or CB300 (commercial box truck) chassis. Some suspension parts are different between models.