I weighed our KZ TT on the way home from the dealer and it weighed 190 lbs more than the sticker on the inside of a cabinet door. We ordered some heavier options (solid surface counters, eg.) and I suspect their published UVW was an average or a baseline figure and they don't actually weigh each unit.
The weight of ALL cargo fully loaded for camping for two for our 29' TT came to only 555 lbs. That's all the usual cargo anyone would have like BBQ, chocks, sewer stuff, outdoor mat, snap-up brackets, kitchen stuff + food, stuff in bathroom, lots of misc. stuff under the bed, some books and DVDs, etc., etc... I would say allowing 1,000 lbs is more than ample unless maybe full-timing, going on a very long trip, have a large family and/or the TT is long.
People give all sorts of figures for the weight of cargo in the 1K-3K range. Some brands and models of TTs have a high CCC and some have low CCC. Using the GVWR is a good thing for shopping to be conservative and weighing your TT after fully loaded is a good idea to know where you're at. The weight of holding tank contents (8.3 lbs/gal.) can really throw things off and in some cases even put you over the GVWR. GVWR is a good number for determining what rating of WDH to get. Modifications & upgrades will add more weight (likely under 200 lbs). If towing with a tank of fresh water, some FW tanks are ahead of the axles and can have an impact on the TW and towing.