A small thin waterbed mattress inside a tent cot that is filled with water that's heated by the sun during the day and cooled at night.
Get this tent cot and water mattress (yes, it's meant for water, not air! It seems it comes from a medical supply company.):
Then build something like this:
The idea is that the sun will heat the water in the coil of copper tubing, and by the thermosiphon effect, heat the water in the bucket.
During the day, the water in the water mattress will be cold, providing a (hopefully) nice place to lie down, even in the summer. Valves A and C will be closed to prevent my body weight from pushing the water out of the mattress, and valve B will be open to keep the water flowing into the coil of copper tubing.
Then, toward the end of the day, valve A and C will be opened and valve B closed, causing the thermosiphoned hot water to flow from the bucket into the water mattress. (Nothing can be weighing down the mattress at this time, of course.)
Now, the mattress is filled with warm water which will (hopefully) keep me warm at night.
Meanwhile, the water in the bucket is still outside the tent and cooling in the night air, ready to be put into the mattress when it gets hot during the day. I'm not sure how to do this part, but perhaps with valves A and C closed, the bucket could be lifted above the height of the mattress. Then, assuming the mattress can accommodate the extra volume of water, the water in the bucket will drain into it. Then valve B is closed, and the bucket is set down. Valve A is then opened, and the bucket will fill with water from the other side of the mattress.
Concern About the Weight of the Water
The water mattress is 32" x 68" x 4". This comes to 8704 cubic inches if it were a perfect rectangular box, which is 37.7 gallons. I think 25 gallons might be a reasonable guess as to actual volume of the mattress. Assuming 8.3 lbs per gallon, this comes to 208 lbs.
The cot is rated for 500 lbs, so it should be strong enough.
Is the design sound? Is there a better way?
Is a five gallon bucket of water enough?
How can I estimate how much the sun will heat the water and the temperatures that can be maintained inside the tent?
How much and what diameter of copper tubing should I use? Ideally, I'd build a solar water-heating panel but portability is a concern, as is cost and labor. I'm disabled with CFS, so I have very little money and energy (for building this, assembling it, and carrying things).
(This whole project is because I want a comfortable place to stay when away from home. I cannot be exposed to mold, and most houses have mold at levels that, though unnoticeable to healthy people, will affect those who have developed sensitivity. I visit a distant doctor and would like to stay with my aunt, setting this tent up in the backyard. It might also enable me to attend an upcoming family reunion.)
I'm thinking I could make a solar cooker out of foam core and plexiglass, which, in addition to cooking food in, could also be sized to allow the coil of copper tubing to be placed inside.