I use a Sherline scale...far easier, but, here is a link to how to use your bathroom scale to achieve the same result. Pay close attention to the ratios; if you get them wrong it will throw off the weight, but, properly done it will yield quite accurate results. It uses leverage to get the result as opposed to the hydraulics of the Sherline, that, essentially does the same thing.
How to measure your tongue weight with a bathroom scale and some lumber and pipe scraps
My 2 cents, your mileage may vary...
2 Cats; J-Lo and Ragamuffin :R
2014 Thor Tuscany 40RX DP
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2011 Keystone Cougar 318SAB (now gone)
2008 FunFinder X 210WBS (Sadly gone)