If you use the GVWR of the tow vehicle as its weight when you do GCWR - tow vehicle weight, then you don't have to worry about how much stuff you put in it. You've already included its max weight in the tow capacity calculation. So, if GCWR - GVWR leaves you with 5000 lbs, you have already accounted for passengers and gear in the tow vehicle and you ought to be able to tow a real 5000 lb (loaded) trailer. No further deductions needed. Weekenders rarely carry more than a 1000 lbs of stuff in the trailer, even if the water tank is full. The tank isn't that large on that size trailer (typically about 30 gallons), so you are looking at maybe 240 lbs of water. Again, if you use the trailer GVWR as its estimated weight, you can ignire the actual weight until you get a chance to weigh it after purchase. At least some of the models you mentioned have a GVWR under 5000 lbs.
The Rockwood 2306, for example, has a trailer GVWR of 4469, so even loaded to its max you are nicely under the 5000 lbs you calculated.
