An adapter won't work to give you 7-pin functionality. Instead you need at least three things:
1) A 7-pin wiring harness and plug made to fit your year/model of tow vehicle. You can get that at
Trailer Wiring | etrailer.com
or other trailer parts supplier. Also the parts department at the dealership for your truck brand probably has what you need in stock.
2) A trailer brake controller installed and wired into your truck. Best is probably the factory integrated trailer brake controller (ITBC) made for your exact truck, but less expensive aftermarket brake controllers are also available.
3) Fuses and relays added to the truck's power distribution box and/or fuse box.
Unless you are good with wiring automotive electrical stuff, then best is probably to have a good RV dealer or trailer retailer install the wiring harness and brake controller with required fuses and relays.