Welcome Ldwebster65 to irv2.
Congrats on your new RV many safe travels for you.
The button your referring to could be the emergency start button in case your engine battery discharged. It connects your house batteries into your engine starting circuit.
If you have ordered correct WH you can go to RV SYSTEMS & APPLIANCES FORUM
and download water heater manual for installation spec's.
You may have one with new WH.
Shut off LP Gas and 12V power to WH and test for LP leaks before firing it up.
There will be many screws around perimeter of water heater and you will need new ribbon sealant around perimeter of new WH.
Enjoy the forums.