When I first started full timing I wanted to put a small trailer behind the 5th wheel to haul our small motorcycles. So I did a lot of digging into towing laws. When I found out the triple towing, (towing doubles for my trucker friends), was illegal in some of the states we wanted to travel in I nixed the idea and went to plan B, then C, then D.

You have to be real careful with total length. It varies by state. 65' is pretty much the standard. You probably won't exceed that, but you do need to be aware of it.

Then there is the definition of "towed" vehicle. The swivel wheel system attaches at 2 points, not 1. One could argue that it is an extension of the trailer and not a towed vehicle. I thought I found a loophole around triple tow laws. Verdict is still out.

In the end I had a rack built attached to the back of the 5th wheel to carry the motorcycle. That worked out pretty well, except that when looking at the distance from the axles to the back of the fifth wheel and thinking about leverage and weight etc. I deduced that having that much weight that far back would be hard on the frame, so I only put one motorcycle back there, I put the other one in the kitchen. Worked for about 7 years.

When we found the Momentum 376 with the little garage in the back we fell in love with it. Motorcycle go in the garage, it keeps them out of the weather and provides a storage space when they are not in use. We get the full use of our 41' trailer for living space. For us it was a win win.

Coworker & his wife I think have the model you're talking about with the garage in the rear under their bed and for the same reasons really like it. Have the bunkhouse and need the living space plus this isn't the time to be purchasing something new.
It's my understanding from the manufacturers website and a few other sources that due to the "fixed" nature of the platform vs a pivoting connection a regular trailer uses this skirts the multiple trailer laws with minimial to no issues. I hold a Class "A" CDL with several endorsements I understand the requirements for doubles/triples and such and occasionally don't mind a spirited discussion so if the mechanical side of things proves out I'm okay with seeing how the brothers in blue respond to it.
You bring up a very valid point about adding weight to the back of the fifth wheel - have the factory hitch back there but so far just one of those hitch platforms for a small BBQ, propane tank, medium sized cooler, and small ladder - don't want to add very much weight that far back.
Basic quick math I came up with:
Total allowed weight rating is 1690 lbs - Payload is advertised at 1200 lbs so 490 lbs is probably close to the tare weight of the platform.
Items I'm thinking of carrying would be 800-1000 lbs so let's say 1500 lbs total weight?
Max 5th wheel rating is 16,800 lbs and I typically run a bit under that.
If the weight distribuiton is 2/3 on the swivel wheels 1/3 on the hitch then I'd be adding 500 lbs to the rear of the trailer or about 3% of the total trailer capacity.
Most of the weight is centered on the axles (dual slides for kitchine & dining/living area), tools are in the basement opposite side of the kitchen and the generator/batteries are all the way forward so the majority of the "mass" is from the axles forward.

But, if it was this simple seems like you'd see a lot more of them around.....
Where is the "Easy" button??

LOL, no easy button for this one. I like the dual point attachment system for the swivel wheel. The one I looked at the attachment points would also flex up and down, necessary for going over dips in the road. Then pivoting on the single wheel looked like it would make it very easy to back up and turn. But it would also make tail swing a bit more of an issue. I caught a travel trailer with my MC rack at a busy gas station, thought my tail was clear and started the counter turn to get into the gas stall, nope, wasn't clear. I never even felt it in the truck, but the angry man at my window make it clear I had messed up.

The other factor that I looked at very carefully was the wheel and its mounting brackets. The roads I have traveled have been very rough. The SwivelWheel is going to be, for the most part, out of sight. I could see myself dragging it down the highway, sparks flying, for a good distance before I new something was amiss.

If/when you get it hooked up please share your experiences with it. As you have probably noticed not too many folks use them.

Despite my growing up around trucks, having a Class A CDL, and in general being referred to as a "good reverser" by a friend from England what happened to you at the gas station is always a concern - doesn't matter how good you are, @#$% happens in tight quarters. Have considered adding the Furrion camera system my fifth wheel is pre-wired for just because of that but so far resisted due to the cost.
The unit I'm considering has 2 swivel wheels which should be more stable and in looking at how it's constructed the components seem to be plenty "beefy" with good craftmanship and I'd add TPMS monitors to the tires to hopefully give me a heads up to any issues. The unit itself seems well constructed - I'm more concerned with whether the 5th wheel frame can take adding this to the rear of it?
Saw a post not long ago where someone had taken a remote camera they didn't use underneath the rv to monitor the tires while traveling and the idea has merit, but I probably won't go out and buy one.