I have a 2016 tacoma, access cab with the long bed, and set it up for sleeping under the topper... I built a platform that rests on two 2x8 boards that fit in the notches on the sides of the bed, and carpeted two pieces of 3/4 inch ply for the top. I bought a futon mattress from ikea for cheap that folds up and happens to fit the dimensions of the truck bed perfectly. I really want to get the truck bed sized paco pad but the futon is pretty comfortable. The platform obviously doesn't maximize headroom, but it's awesome to have your stuff stored underneath and not have to move it when you want to sleep. I use the underbed storage totes with the little wheels on them, they fit great.
I wouldn't likely enjoy this set up as much in the short bed tacoma - the extra length makes all the difference to me, and I'm only 5'8"
As for long trips - I think the new generation of tacomas are very comfy for long trips - but my old car was 13 years old so anything seems nice compared to that
It gets OK mileage for a truck, nothing amazing but fully loaded down with two bikes and a rack on top driving from Colorado to Sedona AZ I probably managed 19-20mpg or so.
If you go this route I definitely recommend the windoors that flip up - I'm kicking myself for getting the sliders, it would be so much easier to load/unload with those windows.
Here are a few pics...