What you are describing "may" be normal for a Sunline roof. Let me explain and then you can look for this.
This post will help show what I am describing.
As a weight savings measure, Sunline bonded the rubber membrane to a thick fiber board they called "bud board". It is not cardboard like a fluted corrugate packing box, it is dense, solid compressed fiber board. Sort of like a cereal box but 4 times thicker.
They then drape this over the rafters. It gives enough structure to carry a snow load and hold the rubber. It is "not" a walk on roof!!! To walk on the roof you need to place a tarp on the roof to not damage the rubber and use 3/8 to 5/8 plywood pieces about 2 x 4' spanned across the rafters.
The rafters are on 16" centers and if you press down, it will stiffen up at the rafter, (about 3" wide) then the span between the rafters it can sag
a little and sort of bounce like a loose drum.
While this is not a walk on roof, it works and as long as there is no leaks, it has not been a major issue. If you do get a leak, and any leak is bad..., then it can wick water across the fiber. Again a leak is going to get the insulation wet regardless of the bud board and wet is bad news on any camper.
This is how I service my roof and AC unit.
Sunline did make walk on roofs as an option. Some folks have them but they are not the norm. If what I describe sounds like what you are seeing/feeling, then that is a Sunline normal thing. While I would of liked to have a walk on roof, now knowing about how they are built, I myself woudl not hesitate on buying one with this roof structure if the rest of the camper is in good shape. The T-264 SR is a very popular model, nice camper.
I would however be purely anal about a 120% leak check all over. This is not the fault so much of the camper, but the prior owner understanding the need to stay on top of the roof caulk and any penetration into the siding where water can get in.
Hope this helps and good luck
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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