Both have their advantages and disadvantages.
If you are going to have a base camp to return to for a couple of nights or more, the ground tent is going to be better overall. The downside is that you have extra dirt to deal with unless you thrown a tarp under it and you have the potential of visits by wildlife. But a a quick setup tent, like the tents eric and snoranger have do make them pretty darn convenient. Plus you don't have a load sitting up high but that kind of tent is pretty bulky. So will take up a significant amount of bed space.
If you are going to be in a new place every night, the RTT is going to be a better choice. The quickest and easiest to setup and take down are the hard shell models. They have the down side of usually being smaller than a bag model but are more resilient to abuse from vegetation on the trail, collect less road dust, you don't have to fight with the bag to get it closed, and the mpg penalty is less since it has a smoother shape and there is nothing flapping in the wind. Of course there is the load being up high that is a negative as well but it is out of the way and leaves more room for gear in the bed.
Both of the above are going to be quicker and more convenient to setup and take down over a traditional staked tent or a soft covered RTT. So, how important is convenience and setup/take down time for you? Which is going to work better for the Vagabond portion of the trip and the camping portion of the trip for the annual meet? Which is going to have more priority for you? I'm guessing the Vagabond portion is going to be night only stays and moving on. Also, what if you do the TAT again? Which will work best for that?
If you had a bigger bed, I would throw out the idea of truck bed camping but that 5" bed is just too cramped to sleep in, even for me. So, that really isn't an option.
You already nixed the idea of an off road trailer. That could have been the answer to both the Vagabond portion and the main meet portion since you could leave it hitched for an over night and disconnect for a more than one night stay. Of course, there is the drag penalty and something extra to worry about clearing something as you are driving along. A small off road trailer mitigates some of that but it could still be a hang up point you wouldn't otherwise have.
You could go the Two-man tent route to save on space and setup/take down time. With practice, you can set one of those up pretty quick and there are models out there that are almost like the Gazelle in setup time and won't require staking down. They can be pricey though and they aren't as small as a traditional stake down tent. Nor as light.
I'm just throwing out food for thought. Whichever way you go is going to be a compromise.