RV owner's manuals are not what you probably hope for. They are usually very general in nature, full of such 'helpful' tips as "use your seat belts" "While the motor home is moving, the sleeping facilities are not to be used." Real details, as to where your battery isolation relays are located or locations of fuses and what they control are missing.
You can check here:
Manuals - 1990 & Newer - RVing-The Midwest Connection
Spend time in the RV with a flashlight, pen and paper. Write down the make and model of everything you find. Don't be afraid to look in closets, cabinets, and compartments, remove cover panels and look behind things. Note any breakers or fuses you encounter. Then spend some cold winter nights using Google to track down manuals for each system you find. I have almost all of mine downloaded as PDF files for reference.
Some websites to help:
Service Documents and Manuals
Owners Manuals
You could also check on line for manuals for other brands to get general ideas about motorhome storage, use, and ideas. It costs a lot of money to have a manual written, vetted by lawyers, etc. They are kept generic so production or design changes doesn't require the whole process be re-done. Many of the current makers of RVs have pretty good access to documentation, especially Winnebago,
Winnebago | RVs, Motorhomes, Recreational Vehicles
includes plumbing, electrical diagrams. Tiffin also has manuals on line:
Manuals | Tiffin Motorhomes
They can get you started and you'll notice a sameness to owner's manuals that leaves a lot of details lacking.