2017 f150 flat tow light kit?

I have used one of these for several years. No messing around with vehicle wiring, just plug into the trailer socket and the light bar plugs into a power point in the toad. LEDs use minimal power, I have never had a battery drain problem. Just remember to put it inside at night.

If you follow the instructions, there will be no power drain unless you use an electrically operated brake system ( e.g. brake buddy or similar) Since you have a DP, I'd recommend a M&G brake kit. Proportional application, operates off the DP's air brakes ( no electric drain) and once installed, it takes only a few seconds to attach when hooking up the toad.
For lights, look on etrailer.... several kits to choose from. Here's one
hopkins tail light kit

Not sure why the power will be on. Read the Ford manual section for recreational towing. But properly installed, the taillights are powered by the motorhome, not the truck. You don’t say what break system you plan to use, but NSA makes one where no power is required at truck.

Suggest checking on line videos including etrailer. I would put the truck in tow mode and measure the battery drain then estimate the additional aux brake drain. A charge line can keep the battery charged if needed. You might need additional equipment for the lights to work correctly.