I've moved many a mattress in my parent's minivan. With the seats folded down or removed and a bit of an angle and/or squishing in a bit you can usually fit a queen, maybe even a king depending on the mattress and van. Box springs are harder, but often still doable, and in a pinch can be easily strapped to a roof rack.

They also have a '93 ranger with the 7ft bed, still chose to use the van for mattresses as often as not, to need to strap anything down or cover them if there's rain in the forecast.

I did a road trip with my wife a few years back and borrowed their Sedona, took out the back seats, threw a "queen" sized air mattress (I'm pretty sure it was a little undersized from a real mattress, but still pretty close) and the mattress was a little squished on the sides but otherwise fit pretty comfortably in the back, we slept in the van for about a week moving between different campsites.

Know what mattresses don't fit comfortably in? The 5.5ft beds a lot of pickup trucks have these days.