K&N filter will not help.
The answer to your "Has anyone" question is yes, very common. THe problem is due to the design of the body of the motor home. Due to poor design water is sucked into the "Tuba" (air inlet).. Workhorse has a kit that can be installed to deal with it, even though they did not cause the problem (The body builder did).
Contact Workhorse customer service, at the number in your manual, they can send you to an authorized service center to have the kit installed.
Since my coach does NOT have this problem, near as I can tell, I do not know if Workhorse will warrant it.. In listing to them speak I got the feeling they were covering it under warranty even though it is truly NOT caused by anything they did, but rather by the coach builder's design. (As evidenced by the fact I do not have that problem, along with many others)
But call them, they can fix it. Very common, Not sure why the place you went to did not know this.
Home is where I park it!