This is what we ran into, as well. We rocked a Maggiolina for years and years. AMAZING tent. Never a bad night, though we did tire of having to get up/get down/get up, to go pee @ night.
Once we had kids, though, our needs changed, and we went back to ground tenting. Now that the kids are a bit older (7yo,5yo,2yo) we're considering our options for a Kings canyon trip this year...
We'll likely ground tent it again, but we've definitely looked at a few options:
-Westfalia van with pop-tent
-trailer tent like a lifetime/jumping jack style
-Offroad version of a coleman popup
-RTT on a trailer with an annex (similar to what you're considering)
-side by side Overcamp Autohome tents.
Good luck with your search. I don't know if you've looked @ Camping Lab, but they've got some fairly large tents available, too. (not sure if they have annexes, but worth a look anyways, and Fernando provides excellent customer service)
It's a tough decision, and sometimes the only way to know is to spend the $$ and see how it works for your family.
"For He so loved the world, that He sent His only son..."
1992 XJ:"Daddy's Jeep" - The "Please don't hit my Jeep again!" Edition.
2009 Ford Edge:"Penny"- "Mama's new ride, and our new trip car"