TCs can work fine for 3-4 adults, if the adults are reasonable folks.
We have a 12' TC, large dry bath, no slides, 2 adults, 2 large teens, on a 3500 DRW. Large dinette for one kid, flipdown bunk for other. We spend weeks/months at a time and have no problem, but we are used to it, and have been camping together since the boys were small.
If you plan on extended trips like we do (Alaska, etc.), esp. with potentially lousy stretches of weather, I wouldn't go with a TC that is too small. Floor space is one thing, seating for hours on end is another, but sleeping must be comfortable for everyone long-term! That means being able to completely stretch out. Lots of dinettes are tiny in length and/or uncomfortable to sit at because of configuration/table/cushions/...
Agreed that you'll never get anywhere in your quest with a 150 truck. And do make a trip to Truck Camper Warehouse, have a look around, and talk to Bill!!