Gadgets

Somebody buy this thing and use it to get the readings, then drive to a certified scale and I want to know how much it is off. Cuz it will be.

There are OBD splitters that will let you plug 2 things into your ODB port.  From what I've heard, they may not work for everyone or for all devices, but I've had 2 things plugged in and working fine on my truck for several years. Here's one for example: https://www.amazon.com/Outzone-Splitter-Extension-12inches-Vehicles/dp/B07V7DPF9F/ref=sr_1_6?crid=HR21RPKOFZH8&keywords=odb+splitter&qid=1564886032&s=gateway&sprefix=odb+s%2Caps%2C406&sr=8-6

TT

The calculator should still work. As far as I can tell the Hensley Hitch still rotates vertically around the ball, just like any other wdh (it doesn't rotate horizontally around the ball). So the distance you put in the calculator for the distance from the ball to the TV rear axle is the same. As far as the hitch weight itself most of it is rigidly connected to the TV when connected to the shank. I don't know how hard it is to disconnect the trailer from the hitch and leave the hitch connected to the shank but if that's too hard then just subtract the weight of the hitch from the trailer and add it to the truck when calculating tongue weight, you won't be very far off. 

Very cool hitch by the way. 

If you want to try the scales and can find a place nearby to disconnect your trailer for a few minutes you ought to be able to get everything you need in one trip and two passes through the scales. First with the trailer connected but the wdh untensioned, the second with the tow vehicle only with the wdh installed. Get all three axles adding one at a time on the first pass, the two TV axles on the second pass (5 weights total). Go through fully loaded for a trip. After that you can get any other information you want from your trailer and truck specs, a little arithmetic, and this calculator. 

Yes, the manufacturer's comments say any suspension mods do exactly that. A calibration process is provided for that purpose. They also state that the bluetooth signal from the device can interfere with other devices in the rig, so most people use it to get the information then shut it off. 

If its not permanent it seems like the same kind of thing could be done directly with a smartphone and app since the phones all have tilt and acceleration sensors already anyway. Sure enough, there is an app on the App Store called RV Tow Check that is supposed to do something similar. Only $2. But it gets mostly poor reviews. 

My issue with this kind of approach is how are you ever going to be sure that you're really getting accurate data? It's a life safety issue for me and anyone else on the road with me.  So personally, I'll stick with the old school approach, I use the scale at my county landfill, accurate and free to use whenever I need to, which is not very often.