The Metric system will not come into wide spread use as long a my DW is living. She does not want it and certainly the government does not want to challenge her. :) :)
Real weight numbers for my 25 ft. TT are; dry 5004 lbs. as listed by the TT maker, across the scales loaded for trip #1, 6200 lbs. across the scales loaded for trip #2 6400 lbs. (2900 Kg) with 700 to 800 lbs. (362 Kg)on the tongue.
So yes you can pick up 1200 to 1400 lbs.(635 Kg) over dry weight with food, clothes, bedding, toweling and other things you cannot live without.
My Max TT weight is listed as 7500 lbs. Since the axels are rated 3500 lbs. each they expect at least 500 lbs. on the tongue.
In the real world, subtracting my measured tongue weight, from my loaded for the road TT scale weight, leaves 5500 lbs on my TT axels. After that I quit worrying about it. I will exceed the max cargo and max axle capacity of my tow vehicle before I overload the TT axels and tires.
As far as I can tell the only purpose for the dry weight is so the commercial haulers know the weight when they haul multiple units across the country.
As far a the RV makers are concerned, they have no idea of what kind of junk you need when you camp; so they just post the max TT weight, the rest is up to you.