I'm one of those that has actually taken everything out and weighed it. I'm an engineer (it's a curse) and even though we were new I just couldn't believe we put 1200 lbs in that trailer. There was 30 lbs of Tupperware (that was not negotiable), and 40 lbs of a new dish/glass set bought just for the trailer. I was able to win the dishes debate and move us to paper the first time the grey tank reached its limit and backed up while she was in the shower. Paper doesn't use up grey water space.
Our trailer came with a fold-down cargo deck on the back that weighed 70 lbs which I replaced with an 8 lb spare tire carrier.
I drain the water heater before we pull out each time (50 lbs). Our fresh water tank/pump loses prime at 4 gallons, so that's 32 lbs I was carrying that I could not ever use. I now carry 2.5 gallons in a jug in the coach for needs in transit.
I created a spreadsheet model based on scale weights that calculates wheel wt, tongue wt, and both axle wts on the truck based on the weight and location of items added or removed. I update this for each trip because different destinations require different stuff. The sheet compares all those weights to the weight limits for tires, axles, chassis, % tongue wt, etc., so I don't accidentally tip over a limit somewhere.
I watch the trailer limits very carefully because that chassis is built to a price point and I consider it the weak link. I don't have that worry on the truck as long as I stay inside all the engineering limits for the components.
For those of you that are looking up the number of a mental health professional to forward to me, and to quote a post earlier this year, there is no D in OC. Working this puzzle is part of the fun/challenge of camping for me.