Good afternoon and welcome to the forums from SW Idaho.
A locking gas cap will have a small sliding plate over the lock to keep it from filling with water but they are water resistant not water proof.
Did you get any paper work with your unit, if so it may be there.
There are no givens as to the layout so the best way is to use a stud finder.
Many people here use the Command strips with hooks or the hook and loop or Velcro.
2016 F350 6.7L LB CC Reese 28K 2014 Chaparral Lite 266sab
"Those who would give up essential Liberty, to purchase a little temporary Safety, deserve neither Liberty nor Safety." 2014 19 days camping 2015 17 days camping201620 days camping