It may or may not be all you need to do. If you've never used the water system since you bought it,
you should verify the summer/winter mode valve positions, as well as check to see if the water
heater bypass valves are in the "bypass" position.
Here's a link to the owner's manuals page at Roadtrek. Maybe you should download your model/year
and have a look, before you fire up the heater. There are pictures of the water heater connections
and valve positions. They're PDF files, so you'll need some form of Adobe Reader on your pc.
It might be that your heater is currently connected to the water system, and will fill the way you suggest,
but you really need to be sure to avoid any damage.
Hope this helps (a bit?