Cadillac 2007 DTS Owner's Manual page 342

Parking on Hills
CAUTION:
{
You really should not park your vehicle,
with a trailer attached, on a hill.
If something goes wrong, your rig could
start to move. People can be injured, and
both your vehicle and the trailer can be
damaged.
342
But if you ever have to park your rig on a hill,
here is how to do it:
1. Apply your regular brakes, but do not shift into
PARK (P).
2. Have someone place chocks behind the
trailer wheels.
3. When the wheel chocks are in place, release
the regular brakes until the chocks absorb
the load.
4. Reapply the regular brakes. Then shift
into PARK (P) firmly and apply your parking
brake.
5. Release the regular brakes.