On my Country Coach Magna, I would try to replace any switches that turn on that blower motor or run cheater wires from the switch to the blower to see if either solution worked.
If you measured voltage beyond the tested fuse, the switch must be ok.
I am not aware of many National RV owners frequenting this forum. So you may get better answers from National RV forums.
1995 38' CC Magna #5280
C8.3L 300hp Cummins, 31,000lbs
Gillig Bus Chassis 05 Jeep Wk