I think the cost of Tiny Houses has a really big, but common misconception on price. That is, they are indeed small, but they are also a home. That means while a kitchen might not be 24 x 24 like in a real house, it is still a kitchen, and so the cost is in having a sink, and all the other trappings. If components used include RV sized stuff, that has a huge cost as well since a
water heater for a RV is going to be 3 times more than a normal sized house since it is a specialty item and not many are sold.
Another misconception is time. Anything ginormous, or small in size takes a lot more time to build, and with Tiny Houses there is a lot of detail work, just as with a real house. That is why in the end a Tiny House costs so much. Again, though small, it still is a complete home....kitchen, bathroom, bedrooms. And to work well, they must be well designed and accurately built.
None of that means your idea does not have merit, or building Tiny Houses could not be profitable. I think there is a lot of hidden costs in building Tiny Houses and the
profit margin may not be as good as it initially appears.
As for using a Tractor Trailer Truck Trailer...that is wrought with problems from the get-go. The biggest issue is they must be moved via truck, and that requires a truck driver with a CDL license. Unlike a 16 foot trailer that falls outside commercial licensing, a trailer truck tiny house would have to conform to Dept of Transportation rules. With truck driver violations costing around $1600 per fine, a truck driver would be very wary of hooking his truck to a Tiny House and hauling it down the road. Such a load screams attention, and truck drivers want anything but that.
But if I sound like I hate your idea, that is not the case at all. I have a trailer truck trailer myself, and have thought of turning it into a Mother-In-Law Tiny House myself. In my case, I would only move it once, so it is no big deal to move, just an excavator trip of about 500 feet.