When I bought my Designer in 2010 I was wondering what the differences actually were between it and the same floor plan in the Eagle. I called Jayco and after getting to the right person, which wasn't a major effort, he emailed me an Excel spreadsheet that showed each difference in a side by side list. The list was pretty extensive and included items I would not have thought about.
I would suggest you contact Jayco and see if they will still provide this information. It was a real eye opener for me to see all of the actual differences and it made the extra cost make sense.
2006 GMC 2500HD CCSB 4x4 Duramax/Allison, Titan 52 gallon fuel tank, Prodigy Controller, B&W Companion Hitch
2010 Jayco Designer 35RLTS, Cummins/Onan RV QG 5500 EVAP, Progressive Industries EMS-PT50C, TST Systems 507 TPMS, RV Flex Armor Roof