We built a bed platform to replace the Happijack sofa in our 2009 23' WW. We reused the angled sofa brackets which on ours were the same as for the bed. Angle iron and plates were used to connect the bed platform to the Happijack brackets with large flathead bolts.
The bed platform was made using 3 sheets of plywood with 2x4s sandwiched between spaced 3.5" apart. The frame was made of poplar boards stained to match the cabinets. It was a big project, well worth the effort and has been working great.
I think the whole project cost around $300 not counting the really nice queen mattress we now sleep on.
I think we got $100 for the sofa from a guy who restores trailers.
PM me if you want more info and pics.
2009 Weekend Warrior, FB2100, 23ft, 7.5k, Maxxis ST225/75R15, Dexter EZ Flex, Equalizer 10k