the implication of being legal when towing is insurance... overweight and they may not pay out in the event of an accident.
I'm with Titch, find a van that suits you then get a vehicle to tow it. Someone was asking a similar Q on ExplorOz (website) and I think they ended up with a small dual cab truck (truck not ute!) to be able to tow the van they ended up with (was big) and be able to take their mutts in cages on the back.
All 5dr Prado's have a 2500kg limit (I think SWB 3dr are 3000kg) and I think Paj's are 2500 except the latest model. Patrol range from 2500 to 3000 (or maybe 3500) depending on engine and gearbox (I don't know the what's what though). Landcruiser Wagons (80, 100series) might be 3000kg but the 75/79/70 series utes and troopys are 3500kg. F Trucks (F250, etc) can tow 3500ish but they won't seat enough for you. Would need to check those weight as I not certain on them
Then there's Disco's as mentioned above, (3500kg??) but like 3.0lt Patrols there's the worry of Landdrover reliability, real or perceived.
But back to the van... why would you only use it when you can't get something else? Can understand staying with friends but staying in a hotel for more than a day or two gets annoying (no facilities... and all yours will be in the van parked outside!!), not to mention the cost.
If I was in your situation I think I'd do this....
1. buy an older van that big enough to :
-- gut it
-- install 2 bunks, make one a triple (that's 5 beds in all). Get them custom made/built in so they are skinner than a regular bed. Run one across the van and one down the side. Leaves room for a kitchen/fridge on other side.
-- make a folding double bed (lift up against a wall which leaves seating around edges and somewhere for the little kids to play in the middle). There are vans with these so it's possible
Maybe have a table that folds down off a wall for meals
-- as much storage as possible (you won't have any room in the car to cart much). I'm think under seating, under double bunk.
-- no shower/toilet... just won't have room.
-- redo running gear, etc
2. weigh van
3. buy 7-8 seater wagon that can tow it.
something like that!