Interesting concept. The distance between the centerline of the two rear axles is worthy of consideration. During a turn, one or both sets of tires
of a dual axle
vehicle has to scrub or slide sideways along the ground. The distance between the centerline of the truck/TAG two rear axles is more like an aircraft bogey than a road vehicle. Tire scrubbing can put a high side load on the tire sidewalls and some aircraft allow for tire castering to reduce this load and the associated tire wear. If the TAG wheels castered then this problem could be reduced but that would add complexity and require bigger wheel wells.
The video on the TAG website shows low speed maneuvers on a wet surface, this would reduce the amount of tire sidewall deformation and tire scrub during these turns. It would be interesting to hear how much tire noise it makes during a tight turn on a high friction surface.
Driving up a ramped surface, like a steep driveway from a flat road, could result in the truck/TAG combination being supported by the front wheels of the truck and the rear TAG wheels, this could put an unusual load on the truck frame, I assume the TAG attach brackets are designed for this loading.
Hooking up the truck/TAG combination on a less than flat surface could be difficult. Getting the blades of the TAG to mount with the truck mounted lock pins may be tricky but perhaps only the left to right alignment is a concern. Maybe the truck pitch is not a significant concern and most RV camp sites are pretty flat anyway.
I hope they do well, it is a good idea.