We did a prototype about ten years ago that might be what you are looking for. We had a client with big Hummer that wanted a large military style trailer that would track his H1 and also wanted a tent system for his family.
Here is what we came up with
I wish we had a picture of this attached to the Hummer so you could get an idea of scale. 4x4 Trailers built a one off huge military trailer and the tent system is two AutoHome medium sized OverCamps. Together they are roughly 7 foot long and over 12 foot wide. There are zippers stitched above the doors so you can simply zip in a small attachment awning between the tents and large awnings or changing rooms on the outside to make the whole rig huge.
These tents are by special order, you can talk to Mike at AutoHome to get them, he will probably want you to sign a liability waiver. The problem with the tents, if you can call it a problem is that they are expedition tents, not designed for family camping. The whole sides of the tent opens up so in you get maximum ventilation in the tropics but if you keep the door and screens open, you could in theory roll right out of the tent in your sleep.
Cost wasn't an object in this concept. The advantage of having two tents is handling. The OverCamps are pretty well optimized for weight but they were quite heavy something like 135 pounds each. So, if you had one tent of comparable size, you would be looking at something like 250 pounds, you would need a forklift to get it up there. The reason why you don't see anything like this marketed is that it exceeds manufacturer's warranties. Even Hummer sets a 165 pound maximum for weight on the roof. You don't have to worry about the structure, these are designed for rollovers. It is the dynamic load, when you are driving, that they are concerned with.