Many RV's with a corner bed use a mattress with a chopped offed or rounded corners for better access or room. There is an outfit in South Carolina who custom makes sheets for RV's.
http://ablifestyles.net/
Do not believe what the dealer tells you, they probably don't know if the mattresses is original or not, and are merely seeking to give you an answer, right or wrong. Looking at the brochure for yours, it appears that some floorplans do have rounded corners at the foot of the bed, while others have squared off corners.
https://assets.lazydays.com/450196_NonWatermarkedPdf.pdf
You don't say exactly which model you have, but the 3171DS, the 3051S, and the 2861DS all appear to have the rounded corners, while the 3101SS and the 3011DS both appear to have the squared corners.
Charles
'03 Ram 2500 CTD, 5.9HO six speed, PacBrake Exh Brake, std cab, long bed, Leer top and 2008 Bigfoot 25B21RB.. previously (both gone) 2008 Thor/Dutchman Freedom Spirit 180 & 2007 Winnebago View 23H Motorhome.