Hi gang,
Making no excuses whatsoever for why RV makers use shorter beds, I think I "may" know why.
Many popular floor plans have the bed placed across the width of the trailer. When this is done, the bed gets constrained by the overall width of the trailer. With a dresser opposite - just across from the foot of the bed, with the bed slide in, there must be clearance at the foot of the bed for the slide to close fully.
I don't have access to my trailer today and I have not measured my Landmark but assuming the outside width is 8' or 96", if I take off about 8" for the finished thickness of the 2 side walls (includes stud depth + indoor & outdoor sheathing), that leaves 88" for stuff.
//heartlandowners.org/images/bed_sizes.gif
If a full king mattress at 80" in length was used on a bed frame with no head board, that would leave only 8" of depth for an aisle way and a dresser.
Even on a Teton with a 102" wide body (and 6 inches for the width of 2 of their walls), that leaves about 96" inside. With an 80" long mattress, you're left with 16" for an aisle and dresser.
My logic above may not be flawless but if it is close, then I can understand why it is difficult to use a full 80" length mattress on a bed that is placed across the width of the trailer.
That all said, I too wish I had a longer bed as my toes curl over the end. But I'd never be able to get around it and I'd have to prop the mattress up to close the slide.
Jim