If you buy a bed at the store, they come in standards lengths and widths, twin, full, queen and king, they go into various size bedrooms.
The RV bed is placed if a bedroom of fixed width so the bed size is adjusted as needed to fit in with whatever else is crammed into it. There may common sizes for RV beds but they seem to be manufacturer/model dependent.
It is entirely possible that the salesperson does not know the bed size of every model. However, when the customer inquires, they should tell them it is an RV queen size etc. which is usually smaller than the standard size bed.
The RV industry does not spend a lot of money on the mattress either. Almost everyone end up adding a pad of some sort.
