I have in other RVs made that conversion. Couple issues will determine your success. Length, do you have the standard 80" that is the residential queen. If not then doing a folding mattress as is used on some RVs will be required. Width of base?? Can the base the mattress sits on be cut to the 60" width? Some RVs will have smaller support base under the actual mattress base plywood making it easy to do. I'm not familiar with your RV so these comments are general in nature. If so then cut the plywood base to desired width and put desired mattress on it.
I'm considering making that modification to my Valencia. It has a bastardized queen, some call it an "olympic RV queen" at 66x76. The actual bed base, fixed with fresh water tanks, is 59" wide. The sliding part of the base the mattress sits on is 66" but it's framed with aluminum square tubing. It's design has extra tubing outside the main part I can cut off and get it just under 60" width. Gonna be a lot of work to add 3" maneuvering to each side. Then I'll get a queen "tempurpedic type" mattress and cut to 76" length.