When I look up inside air box from under griller in 1998 Holiday Rambler. I can see in air box after I disconnect the round hose. I see bottom of my K&N filter and at back of box it looks like a small filter 2x4" approx. Is that a air breather filter that can be replaced?????

 

Shot in dark, but if I understand what you are seeing, is it like a cotton fibre looking filter? If I'm thinking about the same thing you are, that is basically PCV filter. As engine pulls in oil vapor, that filter more or less stops liquid oil being pulled in and makes sure it is just fumes/vapor. You can use the foam fish tank filter material in its place. Just pull old one, cut new one to match, and you're set.