Weighing your unit is a good idea, but you usually do that when fully loaded and ready for a trip. To know weights while shopping, each unit will be different and it will be displayed inside the unit. Use those as very general guidelines. You'll need to have a truck capable of towing whatever unit you're interested in. I usually keep my slides in when not in use, others keep them out. Do some research, here and other places. Use the search function, and ask specific questions. You'll find lots of help here.
P.S. Don't take a dealer at his word when he, or she, tells you what you can tow. They are notorious for being incorrect, at best. Do your research so that YOU know what you can safely tow.