Most players, either RV or auto, will have a way to detach the unit from the mounting cage. It involves the use of a removal tool, usually. Check with the mfg of your player for that detail.
Typical RV headunits are 2 DIN spaces. If you choose a 1 DIN unit to replace it, there are accessory faceplate bezels that will accommodate that. There should be adequate wire length for replacement.
Pulling wire through RV walls or ceilings is problematic. If you don't want surface mount wiring, pulling wire through the under-floor space is probably going to be your best tactic. If heating ducts align well, that can be another tactic.
FWIW, retail stereo dates to around 1948-1950. I use a pair of modified Boston A-40's in my trailer. It is more than anyone needs for good sound. I have no need for a subwoofer.