I would make some changes to your estimate method, but I think you are OK.
"Dry weight" means the weight of the trailer before any factory-installed options are added. For a TM, the only heavy options are the air conditioner and the awning, so you would need to account for their weight. The other options are pretty light, so I might consider adding 100 pounds for all of them.
Water weights 8.5 pounds per gallon, so a 20-gallon tank weighs 170 pounds.
I can't imagine that a hitch weighs 400 pounds. I would think more like 200 pounds, but you should probably check.
Propane weights 4.2 pounds per gallon, and each tank contains 4.7 gallons. Altogether you have about 40 pounds of propane. The tanks themselves are installed by the factory, and so are included in the dry weight.
As you point out, the stuff you load in needs to be accounted for. Think food, clothes, bedding, cooking and eating dishes, TV-radio-computer-etc, water hose, sewer hose, some tools, and so forth. Unless you are really really good at packing light, I wouldn't be comfortable adding less than 500 pounds for all of this.
One item that is always in question is the battery. I think the dealer installs the battery, not the factory, so it is not included in the dry weight. You might want to check this if you are nervous - batteries are heavy.
One thing that most folks miss is that the rated towing capacity of the tow vehicle is the capacity when the vehicle is completely unloaded except for the driver. In other words, if you put 500 pounds of stuff in the back seat or the trunk of your tow vehicle, and add a 150-pound passenger, the tow rating is decreased by 650 pounds. Manufacturers don't like to admit this, but it is always buried in the spec somewhere, usually indicated by a small asterisk in the tow rating.
Finally, you should know if your vehicle has a factory-installed tow package, and if it does not, what is the tow rating as equipped. Probably the most important item in the towing package is an auxiliary transmission cooler. If your vehicle doesn't have the package, you should give serious consideration to adding one of these coolers. At the same time, you should consider adding a transmission temperature gauge. There are many available that simply plug into the OBD-II plug under the dashboard. They are generically referred to as a scan gauge, and Scan Gauge is also the brand name of one of them. There is a lot of discussion in this forum, which you can find with the search tool.
It is quite common to find that a packed-up-and-ready-to-camp TM weighs around 3800-4000 pounds, so you have some margin assuming you don't load up your van. Packing light would increase your margin, of course. The best thing to do is get packed up, ready to go, and then weigh your rig. Many truck stops have something called a Cat Scale, where for about $10 you simply drive on, drive off, and it gives you a printout listing the weight of all three axles - tow vehicle front, tow vehicle rear, and trailer. It is cheap, super-easy, and a very valuable piece of knowledge.
There are many owners on this forum who tow TMs with 5000-pound rated vehicles, and they are generally quite happy. If you plan to go into high mountains, you will notice the load, but it won't be catastrophic. Enjoy!
Bill
