Well, if its a standard cabover, it'll be built a couple different ways.
First off, you're going to need to strip all the interior paneling off in any case, which will allow you to see the remains of the existing frame to rebuild it.
Usually, the material is 1x2s, 1x3s, or 1x4s. Fleetwood might have gone to 2-by material, but I'm not certain.
Since its showing rot by the sounds of it, I'll lean toward it being built of wood. So, the above still applies.
The deck the bed rests on I believe is just a sold sheet of underlayment, possibly two glued together.
'92 Dodge W250 "Dually" Power Wagon
'74 KIT 1106 Kamper Slide-in Truck Camper
'06 Heartland Bighorn 3400RL Fifth wheel
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