welcome to the forums...
this is a LONG covered topic here are just a few of the threads...
many issues to consider.
1. layout (it's very hard to put a QUEEN in a rear bath model)
2. floor space (the isle means more floor space even in rear/front twins)
3. length (twins are generally LONGER beds compared to any a/s (double/queen)
4. making the bed is easier with smaller mattresses
5. twins allows friends to go camping without becoming buddies...
6. twins can serve double duty as day sofas
7. OVER the bed/sleeping area storage configurations differ and are sometimes better in twin versions...
8. older traylas are NARROWER, planting a double/queen may result in 6 inches of walking space at the perimeter
9. some models with double beds trap one partners AGAINST the wall (and some folks LIKE that)
10. and so on.
but most of the PRE 80s models with FULL plumbing have REAR bath rooms,
so twins (or even bunks) can easily go in the center of the units.