Ours is an Onan diesel 7500 but it might be the same. There are two plastic caps next to each other. One for the oil the other the coolant reservoir. I couldn't figure them out at first either. Both have an O ring and just pop off with a tug. It sure looked to me like they twisted off so I scratched my head for a while on that one.
EDIT: Woops. I replied to quick. My caps are right out front, not under a panel so that might not help.
Mark & Nancy
2004 Winnebago Vectra 40KD
Shep dog, R.I.P. Kenzie dog Toad 2015 Jeep Wrangler Willys Wheeler