TL;DR: It works. If you have a 320 and a need to regularly check your tongue weight, it may be worth the money due to ease of use and portability. If you only need to check occasionally, a bathroom scale will suffice.
The long version:
Here’s the thing: scales are most accurate near the center average of their weight range. The lowest tongue weight scale in the US is calibrated for up to 1000 lbs. A typical loaded 320 might max out with anywhere between 180 and 300 lb tongue weight depending on model and how you load your gear (unless you’ve added a bike rack and 2 6v batteries and a tool box on the frame, which will make it higher!)...well below the most accurate point of a typical 1000 lb tongue weight scale.
This British “noseweight” scale was brought up here https://tab-rv.vanillacommunity.com/discussion/10506/are-you-constantly-concerned-about-tongue-weight as a possible option for some 320 owners (not for the 400 as it maxes out at 298 lbs). As most of us know, in Europe, smaller trailers like ours are much more common than here, and it appears that accessories like these, calibrated for our lighter weights, are more common too. In looking over tons of reviews, this Milenco was rated as one of the best of this style of scale, and at the time, it was available via Amazon US and their global sales program for a fairly reasonable price of less than $60 total including shipping and tax, so I ordered one. It has since gone in and out of stock, and is sometimes available on ebay. https://smile.amazon.com/gp/product/B004W17D94/ There exists a larger version (the 3155) that would work for 400 owners or 320 owners who carry a lot of gear, but Amazon is not currently shipping it to the states.
Because it’s from the UK, expect it to take a little extra time to arrive, mine took 13 days to the US West Coast. It’s also only marked in kilograms, at 5 kg increments, so you’ll need to convert if you don’t already think in metric.
Out of the box, the Milenco 2691 is a nice piece of kit. It seems well built, not flimsy at all even with weight on it, and comes with its own drawstring bag to keep it clean in storage. It’s easy to use; simply raise the trailer tongue using the jack until you can fit the scale under the coupler at the ball socket, then raise the jack to lower the trailer tongue onto the scale. Once the jack is completely off the ground, measure the tongue height to make sure it’s the same as when you are towing (16” for my Boondock). In my case, due to a slightly sloped pebbled driveway, I did add a flat 3/4” thick board beneath the scale to get the right height and a more stable surface for the scale. I also checked with a level to ensure the trailer was not tilted forward or back. You’ll want to take off the jack wheel (if removable) and put it where you normally store it in transit for the best accuracy, some of those can be heavy and may throw off your numbers a little when left on.
With tanks winterized (maybe 3/4 gallon of antifreeze between waste tanks and pipes), 2 ActionPackers on the front platform at about 20 lbs total, spare tire mounted underneath and behind the axle, no food, no clothes or bedding, 1 group 27 AGM battery and a half full propane tank, I’m at 84-85 kg, or 185-187 lbs. And my Boondock is actually tongue light compared to most due to 100 extra lbs of Purpleline trailer movers mounted behind the axle.
As far as accuracy goes, this pretty much matches what I found previously using the bathroom scale method, only the Milenco was a bit easier for me than trying to arrange the bathroom scale setup, since for that I needed a just-right sized block of wood balanced on the scale platform to support the coupler at the right height, then had to subtract the weight of the wood, etc. The Milenco also weighs a few pounds less than my bathroom scale, and will be easily stashed in an odd corner of the car or trailer. Plus, since I’m often working with a non level surface, the smaller footprint of the Milenco means I can use a much smaller flat support beneath it going forward, which will also be easy to tote and store. On the other hand, I can’t easily weigh myself on a tongue weight scale! LOL
Review score: 4 of 5 stars; -1 for being difficult to obtain in the US. It definitely simplifies the weighing process for me, but is probably a luxury rather than a necessity for most folks as the bathroom scale method is pretty easy to do for these lighter tongue weights.
(Yes, I’m overdue to pressure wash the carport floor...)