Easy way to determine how much Travel Trailer I can buy?

As has been mentioned, there is "Towing" and there is "Safe and enjoyable" Towing. Exceed the payload of your tow vehicle by very much and you will be back on here asking about tires, expensive hitches, and suspension upgrades. You will learn what "Sway" and "White knuckle driving" means. Weigh the tow vehicle ready to tow and subtract from the GVWR to get recommended payload number. Calculate tongue weight conservatively and see where you stand. Better yet, get actual weights.