I was thinking there is surely no simple answer, especially, if the budget is low. Ideally, an Entertainer bus with 12 bunks would be the best solution and some of those do have a slide or two.
Another thought that was mentioned is to buy a good used class A pulling a travel trailer. Some of the kids must be getting old enough to drive and if old enough and responsible, could drive the van . Yes, it would be more fuel usage but that is a big family and it costs what it costs.
Wishing the best of luck to the OP.
MM.