"IF" you trade, you need a 3500 diesel to equal the payload you wil have with your currect rig. Because as mentioned, the diesel will weigh some 500-600 lbs more than your gas motor. SO in reality, that is a loss of 5000 lbs of tow capacity!
NOt sure how much bigger of a trailer you want, but if a 5w, go to a dually if you have a family! I would even skip the sw3500! My dually has 4100 lbs of payload, my old diesel CC with a9200 gvwr had 2400 lbs. BIG difference in what you can or can not do! A 10K 5w had the old truck at gvwr before adding occupants, this truck I still have 1600 lbs for occupants etc assuming 25% HW!
Marty
Marty