You can leave them both on. The only real benefit is that your diesel burner will cycle less often so you'll save some diesel fuel. If you're getting free campground shore power it's a no brainer. If you are paying for the electricity then that's different. If it's really important to stretch your diesel fuel out or you'd rather not run thediesel burner that much then I'd do it but otherwise you'll find that the diesel burner is a more energy efficient heating system than the electric element. So, from a pure dollars and cents economy standpoint, I'd leave the electric off.
Mark & Leann Quasius
2016 Cornerstone 45A
2007 Allegro Bus 42QRP (Sold)
2012 Jeep Wrangler Unlimited - Rubicon