On the fridge,, ID tag is likely inside the lower door.. That will have model
on the water heater, Outside, open the access panel, likely find info inthere, may be hard to spot however
On the inverter.. Track it down and if needed use your phone to "shoot" (photo) the label so you can read it
There are enough differences we can not really tell
As someone said, a mobile RV tech can show you around plan on 2 hours labor charge at the minimum... May not be that much, but plan on at least that much.