Electric won't keep up with uses as demanding as a shower. Yours is acting normal, Michael. Only the diesel side will keep up adequately. Before hitting the shower, be sure to flip on the diesel side. The electric side is okay for low demands, and is useful to help defer using more diesel than you have to when you've already paid for electric-included RV sites anyway.
You should also notice, especially on cooler days, that your furnace outlets shut down after a few seconds of hot water use, because hot water has priority over space heating, and the system makes sure with the diesel side that hot water is as plentiful as possible.
-Joel