I think jbell pretty well lays out the easiest way to do it. Cut off the weird lance plug and replace it with a standard seven pin like you have on your trailer. Then go to etrailer and buy some version of this 5th Wheel/Gooseneck 90-Degree Wiring Harness w/ 7-Pole Plug – GM, Ford, Ram, Toyota – 9′ Long
Item # 50-97-410. It allows you to tee at the seven pin on your truck and run an additional seven pin into the wall of the bed. I have one in my truck for my fifth wheel and my truck camper.
If you really want to go through the trouble of using the plug that Lance uses, I can give you a part number for what I believe is the correct seven pin receptacle, but that is not the direction I would go.
I have to admit I am really surprised no one you spoke with understood you question as this a fairly common issue with Lance campers in my experience. I expect you could also contact Lance customer support directly and they would be happy to guide you.
Steve and Andra
2012 F350 6.2 gasser SRW LB
Fab Fours front and rear in case we run into a rhino
2019 Northstar Laredo SC