If there's room, you can make a "closet bed" / "Murphy bed". Essentially just hinge the mattress/bed to the wall and lift the other end when not in use.

Much more robust and less finicky than lifting the whole thing up under the roof.

If there's a folding bed variant that suits your needs (sleeping comfort, size etc) then that may also be a space saving alternative.

You might try to find an ambulance body off a dually van/truck. They vary in size but average ones are 12' long, SUPER strong alloy aluminum extrusion welded joints and well insulated. I found one lightly rolled, awsome find for $400, I talked to a few guys who scrap the older aluminum ones and they said it is hard to crush them with a trackhoe. It is skinned in 1/8" aluminum. The one I have is a Lifeline type I or II (dont remember) but I would bet it would cost 40k just for the rear body new. As a bonus there was a bunch of electricals left in it. You could then use tires and axles of your chosing, I am using 17.5" singles good for 6000lbs each on a custom frame.


http://www.lifelineambulance.com/products.html
Ironwood

WV and MD, they didn't put any insulation so the aluminum conducted the heat and it got sort of a greenhouse effect even with the roof vent open.
I thought that if they would put a small window in the bottom of the side door to draw in cool air and let it circulate it would help.
it would also get very cold at night, again the aluminum conducting the heat.
I'd put a layer of the ping solid foam insulation then the aluminum bubble wrap stuff to reflect inside and then a strip of the foam vapor barrier stuff on the frame studs before skinning the inside with wood to isolate from the aluminum frame.
IIRC they just did a 4x8 plywood for the bed with an air mattress sitting on top so you could walk alongside the bed.

There is a guy on expedition portal who bought a u haul body then chopped the bottom off and clamped it on top the bed rails of his truck.

If I could find a place to buy 1/8" x4' x8' aluminum affordable and a bending brake big enough I drew out a design to re do my whole camper in 1/8" aluminum so the outside skin is basically the frame, like the body of a car.
But the cost is more than my budget so I'm using plywood and 1x lumber like the original.

I'd go with an inflatable mattress. Adjustable firmness and much lighter. Packs small

The trailer will be insulated with spray foam insulation which is the same type used to insulate walls in homes. I got a quote yesterday from a company and they will do the spraying for a bit more cost then purchasing rigid insulation panels. He told me it would take only 15-20 minutes to spray the trailer and whereas it would take me some hours to complete with rigid insulation panels.

Dagney provided me with a good link to another forum that has a lot of info on these type of trailer conversions and has resulted in several new ideas to contemplate including a folding bed.

What kind of off-gassing does that spray insulation produce? Does your sprayer guy know what you're going to be using it for? Have you read the Material Safety & Data Sheet on the type they're planning to use? How does heat affect the cured foam?

I googled 'spray foam insulation msds' and grabbed the first one I came across. Others may be different. Here are snippets. I don't know what they are, but some of it didn't sound so great to be closed in with.

*************************
RHH Foam Systems
VERSI-FOAM Systems I, 9, 15, 50 and 1.75 pcf Refillable - Standard Component A

SECTION 2 - CHEMICAL COMPOSITION
CHEMICAL.................... CAS #................. CONCENTRATION
Polymethylene polyphenyl.... 9016-87-9............. 80% to 100%
isocyanate
Containing 4,4 Methylene.... 101-68-8
bisphenylisocyanate (MDI)
(Approximately 40%-50% MDI)
Tetrafluoroethane (134a).....811-97-2.............. <15%

EMERGENCY OVERVIEW - Brown liquid. Slightly Musty. Sprayed or heated material harmful if inhaled. May cause allergic skin reaction. May cause allergic respiratory reaction and lung injury. Avoid temperatures above 105°F (41°C). Toxic flammable gases and heat are released under decomposition conditions. Toxic fumes may be released in fire situations. Reacts slowly with water, releasing carbon dioxide, which can cause pressure buildup and rupture of closed containers. Elevated temperatures accelerate this process.

INHALATION - At room temperature, vapors are minimal due to low vapor pressure. Excessive exposure may cause irritation of the eyes, upper respiratory tract, and pulmonary edema (fluid in lungs). May cause respiratory sensitization to susceptible individuals. MDI concentrations below the exposure guidelines may cause allergic respiratory reactions in individuals already sensitized. Symptoms may include coughing, difficult breathing and a feeling of tightness in the chest. Effects may be delayed. Decreased lung function has been associated with overexposure to isocyanates.

******************************

How does something like this compare to the sheet insulation?

Metal boxes get hot in the sun.

If you had several small intake vents along the bottom and more at the top, you could use natural convection to draw in cool air and vent the hot air.

Inflatable beds are light, but they aren't warm in cold weather. The Murphy bed probably comes closest to ideal.

Sounds like fun!

Sue

Check out this web site for some great info http://myswag.org/forum/index.php?board=28.0

You can only see photos if you register, they are very friendly there and although mostly Australians people are on there from all over. Anything built for the Aussie bush is TOUGH. Most on there are doing canvas based trailers but I'm sure there will be useful info for you