"new rver"....I'm going to reiterate what others said, but hopefully explain it a little better.
You could have one of two issues. The first issue "Ranger Smith" mentioned is that you could be seeing an indicator of low voltage from your engine. When you start your engine, there is a heater inside that heats the fuel to help it start easier. This heater can cycle on and off 3-4 times while the engine is running and will draw a LOT of 12 volt power and make your gauge drop and bounce back. This typically only happens when you're in colder weather, like 60 and below.
The other issue is that not all RV's charge the starting batteries while plugged into shore power. I would suspect that a 2015 Excursion would, but that is something you need to ask Fleetwood. Many high end motor homes haven't come with that feature in prior years and a small charging system had to be added between the two battery systems.
Don & Mary
2014 Newmar Dutch Star - All Electric - 450 ISL
2016 GMC Canyon 4WD Crew Cab