I took my fully loaded trailer to a local weigh scale on the way out of town during one of my summer trips.
The truck, trailer, and trailer axle all got weighed.
Trailer minus trailer axle=tongue weight.
First I weighed truck and trailer. The next step I moved the truck's rear axle just off the scale allowing me to drop the trailer. Then after the trailer alone got weighed I hitched up again( without moving the truck),
pulled forward and the trailer's axle got weighed.
I found the above advice on a rv forum and it really helped get everything weighed quickly and I know I could do this on my own if necessary.
Most people, myself included, underestimate the overall weight of their fully loaded trailers. That put the next important number, my tongue weight, right at 15%.
I read a lot on rv forums and most "old timers" suggest closer to 12-13 percent as that falls between the safe range and also found it to eliminate trailer sway.
This scale really helped as I reduced my trailers tongue weight from 15% to 13%. With a recent project removing my HD base cabinets for lighter storage alternatives this little scale will show the tongue weight to be even less.
My cargo trailer has always towed straight and with no sway. Now I just feel better that things have been moved around and balanced better.

