Something clearly is not right or has been rewired at some point. Fleetwood has wiring diagrams for this rig if you contact them. 5 amps is a pretty strong draw. You can go to the fuse block (typically in a brown or black box near the dinette that also houses the battery charger and 110 to 12 volt converter) and start testing circuit by circuit to find the offending device.
These particular charger/converters of your vintage are known for failing after x number of years, depending on the brand so that is also an item to check out anyway (albeit may have no relation to this issue)
In any case, in addition to this amp draw, it seems that there is a problem with the battery isolator or it has been removed from the system. There should be pretty much zero voltage draw from the motorhome's starter battery from the RV accessories. The isolator prevents this from being possible. Here is an example of one that is available.
You will want to contact a place like Dyers to confirm the proper one for your application.
There are great folks in this site with more seasoned experience than me who will have further insights.