Vehicle based comfortable sleeping is a system.
My pad for 20 years has been the Thermarest Basecamp. This type of pad is available from several vendors, comes in various sizes and thicknesses. It mostly self inflates and the valve lets you set your support level. Ideal for vehicle-based use, too bulky for backpacking.
If you can get to an REI store...most have a display of mats and allow you lay them on a platform or the floor to try out. Personal preferences are so varied...you should try before you buy.
Cots were previously mentioned. I will just say I poo-pooed cots for years, until I tried one. For both tent and truck bed camping they are sublime. A cot gives you the initial support & ventilation space the form the foundation of a comfortable sleep system.
Bedding for me varies on climate. Warm/cool is just a fleece bag. One step up includes one of my wool blankets. Winter camping is a down bag. My fav is a 20F rated one that is a generous mummy cut with the entry zipper on the top, has arm holes/vents, and a tie foot area.