You can download PDF manuals for many appliances.
Get a notepad and write down make and model numbers for your fridge, water heater
and furnace. Then go to the manufacturers web site and look for support or manuals.
A photo of your trailer plug would help but at least give us a better description.
Is it flat 5 or round 5 and if it's round 5 does it have a center pin?
What does the plug on the back end of your truck look like???
Many RVs have no switch for 120 volt and 12volt.
If you have a battery in the trailer and all is working then you simply unplug the
AC cord and the battery should take over for lights and keeping the control board
alive in your fridge and water heater and furnace.
Down load your manuals and read a bit and then ask here what you can't figure out.
If you in fact have a hot water heater then you almost for sure have
a 12v water pump and fresh water tank. There should be a switch in
there somewhere for the water pump but I can't tell you where, sorry!
Welcome to the FR forums!!