Simple test. Turn off House Batteries by disconnecting ground. This will make sure they are ABSOLUTELY out of the picture. Now check outlet for power. If none, turn on ignition switch and check again. Often power outlets are connected through the ignition and if it is now live, it's connected to the chassis starting battery. If still no power then it's connected to the house batteries.
You may have to run a second wire from house batteries only to where you need the outlet if it's live on the second part of the test (ign on).
"A wise man can change his mind. A fool never will."
"You only grow old when you run out of new things to do"
2018 Flagstaff Micro Lite 25BDS
2004 Nissan Titan