Tom,
In our 06 Monty we had the exact experience southbound on I-5, fortunately just 15 miles north of Coburg,Or in a blinding rainstorm. Pulled into Monaco and were advised that the drivers side arm from the wiper motor to the wiper was made 1/4th inch to short in that yeargroup. This could result in the wiper getting blown off to the drivers side and locked at the " 9 o'clock" position. Overnight air got us a new arm from Indiana which turned out to be the same length as OEM. Monaco tech cut it in half, bolted in a 1/4inch section and reinstalled.
It was then that we all discovered the real problem which was a cracked motor housing where the drive shaft comes out allowing too much free play and the resulting blow off to the drivers side with severe cross winds.
Bottom line: have 'em check the shaft length and the motor housing,its a 15-30 min fix once you have the right answer.........................Mike