If you are near an IKEA be sure to shop the "As-Is" room every time, when you get there and when you leave. Stuff is added all day long. I try to be there when the doors open at 9:30.
I got my innerspring futon mattress there for $50, BUT, to fit it I have to remove 3 rows of springs as well as cut down the spring steel perimeter frame. A real pain.
I used an IKEA memory foam futon mattress in a micro-motorhome and it could be cut down with a bread knife.
IKEA is a great source for RV adaptable parts, here is a partial list of what I have used from there, almost all of it from the As-Is Room
Solid wood counter tops
S.S. Offset serving shelf mounting pylons (with scrap cut from above)
single handle faucets (very compact)
Curtain tracks (channel with wheeled trucks)
lots of bedding, duvets, down filled comforters
All kinds of fabrics for curtains and upholstery
strips (Native voltage is 12VDC for some)
S.S. Kitchen Sink (round and deep)
All kinds of kitchen storage stuff.
Much of the stuff at IKEA is designed for compact European houses and is very space saving in RV's.