years ago I did this modification to a 2000 Terry, and put pictures on the internet, every once and a while I still see the pictures come up
I had a short queen, but I am 72" tall (and seem to sleep with my feet pointing down so I keep taking the covers off bottom of bed)
The mattress was 74", but the bed was on a slide where when the slide closed the mattress was up against the chest of drawers so you could not use a longer mattress.
I cut a piece of 5/8" plywood to 60" x 18", and mounted this on the foot of the bed on top of bed platform with a piano hinge so the plywood extended 6" past the end of bed frame. Be sure to use short enough screws so you don't have pointy edges when you lift original bed frame top to access storage underneath.
Paint it to stop splinters.
I picked 18" length because it gave me 12" of bearing surface on the new extension plywood against bed platform when open.
Next I cut piece of 1/4" plywood about 18" x 60".
I cut a piece of firm foam the thickness of the mattress into 6" x 60" and glued this foam to top of 1/4" plywood.
Than I covered foam with some upholstery material that was close to the decor of the RV and painted plywood to stop splinters. I chose 18" because this gave me 12" that slides under the original mattress to hold this piece in place while sleeping.
Once we got to campsite, open slide, lift foot of mattress and flip out 18" plywood base, than slide mattress down. Next we would stick 1/4" plywood under head of mattress with foam on head board side than slide mattress on top of 1/4 plywood so it butted up against the foam, and you now have a 60x 80 mattress (and can use standard sheets).
Because the 6" x 60" foam was at the head board end you did not feel any difference in the mattress because the gap was under your pillow.
And the 12" plywood held up the foot of mattress if you sat on it because hinge was screwed into original bed platform.
When it was time to close the slide, you remove the top foam board, close the hinge piece, slide mattress back up against head board, than we would just lay the 1/4" piece of plywood up at the head board with the foam against the head board with pillows siting on the plywood end, than cover it with bed covers.