I am a single and I have fulltimed for the last 3 years in a 35 ft fifth wheel - Bighorn 3010RE. Since a fiver hangs over the bed of a truck, it may be equivalent to a 30 ft trailer (for maneuvering purposes).
I do not need nor do I want anything larger then 35 ft.
IMHO, these would be my suggestions.
Slldes ! The more the merrier ! Mine has 3 slides. I could not be a fulltimer if I didn't have the slides. Slides seem to add more room then they actually do.
A living room that can accomodate 2 recliners (I have 2 Lazy Boys) and maybe a couch that becomes a bed.
A nice big TV. Mine comes with surround sound which is great because I also subscribe to Dish and get their music channels.
A large shower that can really be used as a shower !
A permanent kitchen table that can double as your computer desk.
The option of a second AC unit.
Have the option of a king size bed if wanted.
I also added in a washer and dryer. That steals closet room so I wanted adequate closet space in addition to the w/d.