When we had a popup camper, and wanted to know the tongue weight, I set a jackstand on our bathroom scale (don't tell my wife!). As I slowly lowered the coupler onto the jackstand, I stopped when I could see it would exceed the scale's capacity (300 lbs). At that point, I found a block of wood the same height as the scale. Put 2 of the jackstand's legs on the scale, and 2 on the block. Scale reading x2 = tongue weight (which was about 370 lbs).
There are various ways to create lever-arm scales, which will allow you to measure even higher loads with a 300 lbs bathroom scale. Google if needed.
thebrakeman ('70), DW ('71), DD ('99), DD ('01), DD ('05)
2004 Surveyor SV261T (UltraLite Bunkhouse Hybrid)
2006 Mercury Mountaineer V8 AWD Premier
Equal-i-zer WDH (10k), Prodigy Brake Controller