Consider constructing a simple and crude balance type scale. You would need
a fulcrum, a stout plank or piece of steel and some free weights. Set the
desired weight on one end, fulcrum in the center and set the tongue on the
opposite end. Adjust axel position until the scale is more or less balanced.

We're building our trailer and are in the process of deciding
where to place the axels so that the tongue weight comes out
to 400 to 500 lbs. Our bathroom scale only goes to 200 lbs
or so. And we can't move the trailer & boat to go use the
truck scales at the local yards. Looking online, scales that
go to 500lbs are $600 or so.

So, any ideas on how we can do this measure and/or where to
find (buy or rent) scales without spending too much $$$?

Thanks,
Gary