The only thing really in the way of adding seats in the back is the HVAC system. I used the front seats out of early 2000s chevy trucks. They have built in seat belts. I put them in so that they can still slide back and forth. The tan seat in the picture sits in front of the HVAC and the other seats can slide forward so they are all in the row. There is an incredible amount of room behind the seats for storage. I paid 50$ apiece for the seats. I tried to get all matching, but beggars can't be choosers.
I have heard that some people use sprinter van seats because they can get 5 in a row back there. I have nothing against that, but I think it is a poor choice unless you really need 5 seats. Kids grow. I'm a big guy. Not huge but I'm 6 ft 245. I sat in a sprinter van seat once. Not very comfortable. There are high school kids bigger than me. I would not expect them to sit in that little sprinter van seat on some long trips.
I used 4x4 tube to block up the seats, so it is comfortable for adults to sit back there.
Not the best pictures. I dint realize I had oil on my phone when I took them.
I used 1/2 bolts to bolt them through the floor. I made some large washers out of 1/4-inch plate to put on the bottom. They are 3-4 inches across. They are that big because if we were in an accident I don't want the bolts to rip out.
This is in a 03 freightliner. If I remember in my Peterbilt the HVAC didn't sit back in the corner. I think it was on the driver side under the front of the bunk. That may be a challenge.