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