Remember the old adage: "If it ain't broke, don't fix it."
I'm sure your RV came with a bunch of manuals supplied by the appliance manufacturers, right? In these manuals you'll find the answers to your questions. No manuals? Get the make, model a serial number from those appliances and get the info on line.
That being said, here is a generic recommendation for maintenance.
A/C - clean inside air filter
Fridge - clean interior
Water heater - drain and replace sacrificial anode as needed (if so equipped)
Furnace - clean out the floor vents as needed.
In other words, if these appliances are working, they don't need you to mess with them.
My 2 cents,
Teach
