When you jack up the camper and spin the wheel by hand, can you hear a faint or strong feeling of it? Is it a full 360 degree of rotation or only partly around?
If it is rub, it has to leave tracks of some kind. Look for witness marks of a rub or metal shavings, every where on the backing plate, frame, tire, drum.
Are you sure it is not a tire or rim rub? Again look for wear marks.
Does it get louder with speed? Or does it get louder after towing so many miles? (heats up and then is louder)
You say squeal, is this like a hitch pitch sound or more like a growl of grinding?
If you jack it up and grab the wheel, can you rock the wheel? You are looking for excess bearing play/wobble.
Can you track it down to a wheel? and are you sure it is that wheel?
Does that one wheel get hotter then the others if you feel for heat at the hub from a normal stop?
This sort of falls into 2 areas as the more likely causes,
Something is rubbing or something is vibrating.
Hope this helps
John