You haven't stated the brand(s) of your water heater and furnace, but if they are Surburban, you can get owner's manuals from them. If you have Atwood, do a Google search for them.
Airxcel | Suburban Manufacturing Service
How do I get an owner's manual for my water heater?
You can contact Suburban via written correspondence, phone, e-mail with your model and serial number of the appliance. The Manual can be e-mailed as a PDF file or mailed out in printed form.
We're easy to reach, so don't hesitate to call if you have questions or concerns. Use the phone or fax options below.