Homemade Truck Camper Dolly - Advice Needed

Grit dog wrote:
So the frame is 49" wide? Basically the same width as the camper.
2x4 frame will be fine. You're transferring most of the load through direct compression on the outside edges. Use 2x6 band boards held flush on the bottom if you want a lip on the sides.
If the band board sits outside the frame you can always block between the joists right over the trailer frame if you're worried about compression on the joists.
Save the $ for joist hangers. Not needed.
Assume you're going to sheet the top of the deck?

Thanks Grit Dog, perhaps I didn't state it clearly, the frame bed I want to build is about 49" wide ... about the same width as the truck camper bed. The inside, wheel to wheel measurement of the wagon running gear is 54" ... plenty of room for a 49" frame bed. And it's narrow enough on the outside measurement to fit nicely in between the truck camper box and the outside walls/wings.

I realize all of the serious weight will be held on the outside edges of the truck camper basement. I was just worried that a doubled up 2x4 or even 2x6's wouldn't be strong enough to carry the distributed 3,000# ... there is a about a 10' span on either side that the wooden frame has to carry the load.

Your idea of a taller band board to create a lip against the inner board is a good one. And yes, I plan to put "decking" in for both structural rigidity and to keep critters from access the bottom of the truck camper easily.