I've got the solution. We too had this problem.
I went and bought some plastic insulation material that goes underneath the beds, directly under the mattress, and allows are flow out, but keeps warmer air in. Found this stuff in an ad in the back of Trailer Life magazine.
It was designed for marine use.
It also has another benefit in that it makes the mattress a bit firmer.
You can find this stuff at
www.hyperventmarine.com.
I got 20 feet to do both full sized beds in our model 2619. It cost me about $170 bucks with shipping. But it's light as a feather.
You might check into it.
Otherwise, try one of the skirts for a 5th wheel. I checked into it once because I thought it would provide some additional storage space. TrailManor does have a skirt they can send out. But it looks like those shown in Camping World catalog for a 5th wheel.
Hope I helped!
Gregg in Portland