There's been lots of discussion about deflection. I did some messing around with some material I have on hand.
My testing was very basic. I spaced 2 chunks of 4x4 on the ground 60" apart. I then used my 225#'s to put pressure on the approximate center.
A standard 2x4 dimensional lumber placed flat deflected, but held my weight while I bounced up and down in the center.
A standard 2x3 dimensional lumber placed flat deflected a bit more, but also held my weight while I bounced up and down.
A P1100 1 5/8" 14 gauge Unistrut channel supported my weight as I bounced up and down with no real deflection.
View attachment 68494
A Unistrut P4100 Unistrut on edge held my weight with some deflection while bouncing.
View attachment 68495
A 1x2 aluminum rectangle tube placed flat felt like it would bend before I put my entire weight on it, but it did take quite a bit of my weight. When placed with 2" edge up it supported my weight with bouncing and didn't bend.
A 1 1/4" garage door support steel angle bent with my weight on the center. No real surprise with as light as it is and no decking to distribute the loads.
Keep in mind that the above crude tests were with one only cross support and me putting weight on center section. Including multiple supports in your bed design with 3/8" or 1/2" plywood or particle board decking adds lots more load distribution.
I knew that the 2x4 used flat would be strong enough. To me the 2x3 was a bit of a wild card before my testing showed that it had sufficient strength.
Either style Unistrut set to the 1 5/8" dimension being vertical would be plenty strong.
I didn't have a 1 x 1 1/2" aluminum rectangle to test, but based upon how strong the 2" stock I tested was, I believe that a 1 x 1 1/2" aluminum tube on end would be strong enough.
Using angle stock does require a bit more caution depending upon how heavy the gauge is and how long the leg of the angle placed vertically. As Inertiaman pointed out in another thread, angle does have some advantages as to bolting and fastening if that is the plan.
For the occasional camper.
The cheapest cross supports to purchase would probably be the 2x3 dimensional lumber. By being placed flat the cross supports have only 1 1/2" or so taking away from under bed storage height. My concept with 2x3 or 2x4 lumber would be to set them into notched 2x4 or 2x6 side rails mounted to the side walls.
I would use 4 ea cross supports and 3 each deck panels. The outer deck panels would be 24" wide. The center one would be 32" wide for a total of an 80" long platform. Using 2 ea. 24" panels should work well for one 4x8 sheet good.
To ease installation I would fasten 2 each cross supports to the outer deck boards leaving cross support ledges for the center deck board to sit upon. The 2 end deck boards would set into place followed by the 32" center one. After a few deck screws are installed to keep things in place, you will have a simple bed support ready to use.
Keep in mind that this is a system. During my crude tests the stand alone flat 2x4 and 2x3 flexed with my full 225# on the center of the 60" span. The bed platform installed with decking as I suggest will not have noticeable flex during use no matter how active you may be.