The "users" manuals are worthless, they contain zero information on how to use your RV's appliances or where things like breaker/fuse panel is located, no electrical/plumbing drawings or diagrams and so on.
If you are looking for that type of info, you are out of luck.
Now, if you were wanting to know how to use the appliances like stove, water heater, furnace, fridge, A/C then what you want is the individual manuals which the manufacturer of your RV would have simply shoved loosely into the "user manual" that was lost.
So, what you do is look at each appliance, find the manufacture name and model number, write that down for each item.
Then do a Internet search for each one of those appliances using the manufacturers name and model number and add the word manual at the end of the model number..
For example, my RV has a Atwood G6A-7 pilot water heater..
The search string I would use is-- Atwood g6a-7 manual
That returns
THESE results..
Inside those results I see
THIS from a website called Manaulslib.com
The appliances are common items used across all RVs.