The fridge, when in use, whether on electric or propane needs to be level, within 3 to 5 degrees! Of course the closer to true level the better. I have always traveled with fridge on, as stated you are moving and the fluid gets josseled around as you drive. As far as "the sweet spot" to drive at, you will find it by driving! Each rig, including how much is stored inside will be a little bit different. One thing to keep in mind is the V10 likes to rev. I had one years ago and it was a little scary until I just accepted the beast. 55 on an interstate is a good place to start.