Thanks for the replies. Unfortunately, I just noticed that BCS delivered the coach without a filter in place so I'm sort of swinging in the wind. Looking down the return air vent I can see the frame for the filter and it's about 18" wide so I'm thinking the 10X18 is about correct. Access to it is via a cover above the air inlet ducts for the outside fan.
Of course the temps here in Lebanon, OR are at record highs so I'm running the AC without a filter and need to resolve this issue without excessive delay. Just to add to the drama, I bunged up my left arm and that is the one I need to access the filter cover.
Fred, what is this stuff you were referring to that you cut to size??