There are (or uesed to be) a couple of "Generic" books, as I recall "The RV Book' and THe RV Owner's Handbook,, These are generic covering many RV's.
As Mr. Drake said... The appliances including furnace, water heater, converter, air conditioners and such, The manuals are provided by the maker of the device, MANY of them can be simply downloaded from the company support site.
Owner's manuals for the RV itself, are usually only found at the manufacturer's print shop or publisher, not on line However save for the floor and walls, which are kind of like the ones at home, and the body (Think automotive) there is not a lot of difference rig to rig, in fact, Even the door, on many RV's. is a 3rd party device with it's own owner's manual (you should not need that one).
If you have a motor home, the engine and drivetrain have their own manuals, you get those from whomever made the chassis, not the "House builder".
The infomoration I really need is "Where does this wire go and how does it get there" or sub pipe for wire,,, and ... that's not in the owner's manuals, that's in the SHOP manuals..... Sometimes.