If the shelf thing is an absolute, there's nothing stopping you from a bit of probing. If the studs are wood then they used nails or screws to fix the shtrk. An inexpensive nail finder, (swinging magnet for a couple bucks) from your local HdWr or Lumber Yd will locate the studs for you.
If wall depth is important punch a hole large enough to shove a Kabab rod or sacrifice a coat hanger. It's possible trailers use 3/8 shtrk, (don't know code for them) the hole will be small enough to spackle.
The important thing to bear in mind if you procede is to locate all outlets on both sides of the wall to ensure that you don't hack into one. Depending on the age of the trailer especially if the studs are 2X3s there might not be a shield covering cable running through the studs and a screw or nail could stab the line.
If you have a friend in the construction trade maybe they'll lend you their multi scan stud finder, (electrical, metal, wood and density scans)
Never bite the hand that looks dirty!
The more you know the more you're worth