Here is the equation I use:
Remaining payload divided by .115 or 11.5%
So look at your trucks door sticker to find you payload limit, then subtract the weight of what will be in the truck when towing, i.e. you the wife the kids, dog, etc.
Also subtract the weight of any add-ons to the truck, i.e. cap, bed cover, step boards, etc.
This will give you remaining payload.
Truck Pay load = 1500#
Wife Kids etc = 825#
Remaining pay load = 675
675/.115= 5870#
This is in theory the max trailer "GROSS" weight you would want to purchase, i.e.GVWR.
I always suggest buying something in which you can tow its gross wieght, as in most cases you will find that with in a year or so of use that your "collection" of camping equipement along with food, clothes, bedding, etc. will get you very close to this number.
One area that you can fudge is with respect to the water tanks. As water is 8+lbs per gallon (30 gallons of water = 240+lbs), gray and black water will be more. So if you are running tanks empty there is a bit of GVWR that you can deduct (or actually add, i.e. GVWR for trailer is 6000# but you plan to never run wet to could take the %870# in our example and add 600-700lbs assuming you have 30 gallon water, as well as 30 gallon gray and black water tanks), but you will be pushing the limits if any of these have any real volume while towing.
Note: 11.5% to 13% is the the average tongue wieght for a travel trailer. Toy haulers start with higher "dry" tongue weight but level out as you add the toy weight, i.e. toy haulers dry tongue weights will be in the 17%-20% range, but the percentage decreases as you add proper topy weight to the rear of the trailer. 5th wheels run around 25% of trailers weight on the pin.