Wherever the spring hangers are attached to the frame is where the frame rails are. Don't know about a Teton, but you should be able to move forward from the spring hangers and find a spot for a bottle jack.
Or, you could get a 4x4 that is about 80" long, place under the front of the trailer behind the landing gear and use the jacks on that.
To me, the term "twin I-Beams" just means the steel frame i-beams that run the length of the trailer from the rear to the front overhang.
If your underbelly is covered, then the cover will be attached to the "Twin I-Beams" by sheet metal screws, and that is a way to trace the rails.
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired