Mattresses are more varied for nomads than those in sticks-and-bricks residences. Due to the unique constraints of beds inside a vehicle, mattresses must often move or change each morning and night, and may need to fit into non-standard spaces. As such, conventional mattresses may be less common than custom-sized ones.
Mattress thickness
Choosing a thinner mattress saves weight and space, both of which are valuable in such a small space. If you can, try a thinner mattress than you would have in a sticks and bricks residence to see if you can still get a comfortable sleep. However, good rest is extremely valuable too, so don't skimp on thickness and comfort if it means you'll be tired or hurting in the morning.
Mattress material
If you get a memory foam mattress, note that it will be much firmer in cold temperatures and much softer in warm temperatures. If your vehicle doesn't have adequate heating or cooling, the changing firmness of your mattress may bother you. If you have a heated mattress pad or bed warmer, this may mitigate this problem. Note that you should flip and rotate your foam mattress periodically so it doesn't develop a permanent compression spot where you sleep.[1]
Most people don't like air mattresses suitable for long term use; they are generally less comfortable and less durable than a conventional mattress, and require regular re-inflation as the air slowly leaks out. You may find them useful for temporary use, such as for a guest bed. If you use an air bed during cold weather, you'll also want additional insulation underneath you, such as an extra sleeping bag or blanket; the cold air in the mattress will keep you colder than with a conventional mattress.
Mattress bridges
If you have multiple cushions comprising your mattress and you find yourself slipping into the cracks between them, consider putting a mattress bridge between the sections. It may also be sufficient just to put a fitted sheet or fitted mattress pad over all sections.
