On my 2004 Explorer I have to leave the Key in also.
Since I have the Limited Model it has the keyless entry with the key pad on the frame behind the door.
We sometimes lock the door with the key inside and use that system to unlock it. You could also just lock/unlock with the keyfob and leave the key inside
Most of the time however we just leave the key in.
With mine the battery drain is minimal and I have never had a low battery or need to pulled any fuse even after towing for several days at a time.
My towing lights use no power from the Explorer and my braking system uses minimal power (air system).
We like it as a tow vehicle since it is real 4X4 --- we use on some easier 4WD trails, it has a third row seat for when kids and grandkids are there, and it has a V8 (with TOW PACKAGE) and 7000 lb tow capacity to tow my boat at home.
We have towed ours for about 30,000 miles, including our 15000 mile Alaska trip, with no issues.
2007 Tiffin Allegro Bus 40QSP
2010 Ford Explorer 4WD -Toad
carrying 14 ft. alum boat with boat loader