The quickest fix is placing 1/2 inch plywood on the worn frame under your new mattress - distributes weight and prevents sagging. For long-term, consider replacing with jackknife sofa or complete sleeper mechanism. Plywood solution costs under 0.
Thor Avanti renovation is very doable. Jackknife sofa can replace dinette, vinyl plank flooring is popular, and mattress can be replaced with standard size in most cases.
Replacing recliners with a sleeper sofa in a Class A requires proper mounting brackets, weight considerations, and travel-safe mechanisms. Focus on RV-specific furniture with reinforced frames and secure locking systems.