I just received a "Haul Gauge" and am testing it. It claims to provide a lot of information about your Tow Vehicle Weight, Tow Vehicle Payload, Gross Combined Weight (TV and trailer), Hitch Weight, Pin Weight (if you have a 5th wheel), and Weight Distribution.
So far all I have tested has been my truck's weight and it has returned an accurate weight--or at least close to what the CAT Scale says. This could be handy as you are loading the trailer for a trip and want a running indication of your tongue weight as you load and as a check to ensure you haven't overloaded anything.
It works on any vehicle with an OBDII port and an automatic transmission.http://www.haulgauge.com/
(Added tool name to title for search purposes. - Sharon)