I don't know why those are the only options. The average F350 can be optioned with a dual cab (TONS of room for 4 adults) and a ~3500lb payload (remove the truck bed, and you'll gain another few hundred pounds). There are plenty of trailers that can fit four adults, and that come in similar (and lower) weights, and that's including the frame, axle, suspension, tongue, wheels/tires, and other components that wouldn't be necessary. Seems to be it's a question of coming up with a good layout/design, and being clever about materials used.
Look at the Opus OP15 trailer: queen size bed, dual cots, tons of interior storage, shower, etc., all in a footprint that doesn't seem to be all that big. Put a queen bed over the truck (cabover), make the rear end of the camper slide out like on the Opus, and put two bunks there.
Better yet, copy the design idea from the Bruder EXP-6 and replace the bunks with a couch that turns into cots at night. You've now got your sleeping situation covered, and still have plenty of room left for a toilet stall and an interior kitchen.
There's also the Wingamm Oasi integrated campers that have clever features such as the main bed dropping down from the ceiling (
https://www.motor1.com/photo/5867763/wingamm-oasi-540/). Lots of clever designs out there.
Sure, I'll concede that a slide out is not great for sub-freezing temperatures, but it can be done well and not be a deal breaker. My last trailer had a pop-up roof with the gap filled by canvas-like material, and it worked just fine in -15C, so a tiny perimeter gap around a slide out can definitely be made to work.
The point is, you don't need a 30ft box on a firetruck to go on a two-month road trip. Some unconventional design ideas can be incorporated to make a much smaller, much lighter vehicle perfectly useable and practical.
