I was told, when I bought my first Class C in 1997, that running the WH on both was fine.
When I bought the current rig, the "tech" (PPL) that did my walk-thru said not to run on both, it would "confuse" the control board. I thought...well that s-o-r-t of made sense. But then again, it really didn't when I got to thinking about, it's two, totally different "circuits" if you will. I really couldn't see how one could confuse the other one.
This last trip out though, I ran the WH on electric. We took showers back-to-back and never ran out of hot water.