Frank,
First of all you need to define the size of a double bed. In a store this will mean a minimum size of 6ft 6 inches by 4ft 6 inches. A king size is a 6ft 6 inches by 5 ft., In a caravan/motorhome it will mean whatever space is left for the manufacturer to squeeze it in. Yes, I know many manufacturers stick with the traditional dinette at either end that is designed to be the main bed, but the size can often 'shrink' with development. Some motorhome manufacturers in particular, think a 3 ft wide bed is a double. They must be pygmies. Of course the manufacturer never has to sleep on the product, that is for the customer to enjoy. I only bring in motorhomes to the dicussion, because many of the manufacturers make both products.
Mention has already been made regarding cushions, and this desire of the manufacturers to put rolls at the front edge. In no respect can rolls make a decent bed unless you turn them over, and this can be a major problem if the cushion is over 6 ft long. No wonder the fixed bed idea is getting stronger. It is not just the comfort, it is the ease of assembly. Plus you will in all probability be getting a sprung mattress, not foam filled cushions.
In my mind there is a directly inverse proportional relationship between age of customer and enthusiasm for fiddling with cushions. Define the market and prepare accordingly.