Had a similar situation last year, but in our case it was rats. There are vacuum lines that run as you describe that control the heater vents to switch from defrost to floor vents. You may want to check if yours still work.
End of the day tried to find a schematic, (did find some online, but could not read them) with little success. Called around found a shop that seemed to be pretty familiar with the issue. Couple hundred bucks later, good to go.
As a side note, which you probably are aware of, the MH is 2003, but the chassis is going to be 2002
Good luck with it