Spent many nights in our pickup's bed under a cab high and higher canopies.
Did not spend much on my lash up but sure enjoyed it.
Two sheets of 1/2 inch plywood. Cut so they will fit across the bed above the wheel wells.
Then more 1/2 inch plywood cut so it will support the deck. Deck to truck bed next to the Wheel wells and Deck to truck bed on the edge of each deck at the center of the truck bed.
Do not join the center as it should fit sung but allow sliding out one side at a time so you can use the truck without it. To make it a little stronger I added ends at the forward end of these. No ends at the tail gate.
I used home center angle aluminum metal 1/4 inch thick cut in small lengths and bolted with carriage bolts to hold everything together. Secure with a little flex is good.
On one side I built a 1/2 inch plywood open top box that ran full length of the bed and between the deck supports. Stored fishing poles and other stuff here. The box could be slid out to use.
The other side had 4 plastic home center totes with lids, stored food and clothing in these.
Bought one piece of 3/4inch Electrical conduit and put a two foot L bend on one end then slid it in first with the totes after, pulling the conduit would pull the totes out.
On my longbed F150 a queen sized bed would fit on the deck. We used an air mattress and foam pads I liked the foam pads best.
The lowered Tailgate made a great place to cook and eat from. Yet with canopy and trucks tailgate closed we were snug even in the rain.
Sorry I don't have pictures, and a word of warning using this resulted in our wanting and buying a T/C.
Former user name: "Lots of Stuff"
2014 Jeep Patriot
Spent many of nights in college in the back of my truck (Nissan). Used to camp out in the parking lot of a YMCA I worked at starting at 6:00 or college parking lot. Bars until 2:00, short sleep, and ready to work. Also started camping / traveling with the wife when first married. Everyone loved when I slept over at work...I had the coffee already when they arrived. I took plywood shelf and placed it on 1/3 of the bed where the bed / cap met towards the cab. Under the shelf I placed some camping eggs shell type mattress which I slept on. This way I could sit up. Shelf was good for bags of stuff when sleeping. Cooler went into the front seats. By sleeping on the truck bed, I could sit up easily. A cot would have limited headroom.
Keep your eyes and ears open...You may be able to pick up a small TC for a good price. I love my tiny popup and I can stand up, cook, and easily reach a cold beer in the fridge.
Having done the platform and airbed thing, I would take a good look at the Camper Cots from Cabelas. It looks simple and quick, which allows you to use the truck as a truck if needed, at any time during your travels. If you do decide to build a platform, make it high enough to slide your cooler, and other gear under it. Most importantly, have fun.....