Any manual (if it exists at all) is likely very generic.
This link takes you to a 1993 Lance manual for a general idea:
https://www.lancecamper.com/docs/1993-owners-manual.pdfThe "interesting" information is usually in the individual manuals for the different appliances (water heater, furnace, refrigerator) provided by the manufacturer. Most of these are available online if you search with the ID of the appliance in question. A good starting point might also be the collection at
http://bryantrv.com/.
If you have specific questions about any of the camper systems, you can always ask here.
Have fun with your new-old camper. Spend some time checking all the seams and re-caulking them!