I bought two or three 2xs's and made my own supports under the bed. I'm 6-04 250 and my wife was pregnant when we bought this thing, so we weren't exactly light. We also tossed the junk Denver mattress and bought a much heavier memory foam mattress. I wasn't pleased with the crappy design that Jayco came up with.
The support at the front has a 2x4 plate on the bottom, three vertical studs and a 2x4 plate on top. The bottom plate is screwed into the floor to keep it from falling over when bed is up.
The support about halfway down is sistered in to the 2x2 structure that makes up the bed surround. There is a 2x4 top plate that ties in to the 2x4 verticals.
The 2x2's that run underneath the plywood bed frame rest on top of these little walls and provide solid support to the bed.
Total cost, about $7. The support up in the front allows me to store some longer items (beach umbrella, longer pieces of wood, etc) in front of it, so I don't feel it cuts into my storage space. The support halfway down, I intentionally didn't add a center support because I wanted to be able to store chairs underneath. I still have plenty of storage room, even with both those supports installed.