Hi Steve,
I had a 2004 T2499. Great camper and floor plan. We have no official stud location but can describe where to look on the camper.
As Tommie stated, a stud finder scanning the wall is a place to start with. The large rear window about takes out a lot of the rear wall.
Where on the wall did you want to mount the shelf? High or Low?
At the very outside of windows there "may" be a stud. There is for sure a 1" wide framing piece on all sides of the window. At the very corners of the wall where the rear wall meets the side wall, there is a lot of wood buried in the physical corner. BUT... there is always one of them... you have to screw the screw in on a 45 deg angle to get into that wood as the side wall covers up most of the corner stud wood.
See this pic of mine. I added a coat rack board (3/4" thick pine) up at the top of the window area. The top left in this pic. The corner was screwed in at a 45 angle into the corner as there was nothing straight on and at the widow side I could screw straight in but remember there is only 1" starting from the inside frame of the window
Odds are high there is a stud on both sides of the window floor to ceiling to hold up the wall as there is no other place on that rear all to put one.
Gently taping with a hammer "might" help sound out where the stud is. Just do not tap too hard. The wall board is only 1/8" luan.
Hope this helps
John
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Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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