Normally, they are put on the front cap near the corner, and on the side near the same level. This way, you can look at both levels from the same place. Be sure that you have the trailer actually LEVEL before you put them on...using a tennis ball on the floor in the center of the trailer can help you, or a golf ball, or ball bearing. If there is any lean in any direction, the ball will quickly tell you. Another check is the bathroom door and if it stays wherever you put it during the swing. Some use a level in the freezer compartment also.
Lots of ways to figure it out--bottom line, if you are comfortable in the trailer, its level enough....
'05 36TK3 Mobile Suites
'10 F450 new to me
'09 F450 Lariat 4x4--died