Why does every RV mattress feel wrong even when it looks like a normal size?

I got an RV awhile ago and figured a queen mattress was a queen mattress. Not exactly. Most RVs are actually a "short queen", same width as normal but 5 inches shorter. There are also short king and split sizes for double firmness.

Manufacturers build around cabinets and slide outs rather than standard sizes so people end up with feet hanging off the end or sheets that never fit right and just assume the mattress is of poor quality. Did someone realize mid trip that their mattress was the wrong size the whole time?