Best sized rv for GMC Sierra 1500 gas w/5.3 Litre engine

Pay more attention to the payload capacity of your 1500 - and understand what contributes to it. Payload will be reached/exceeded before towing capacity. You have 4WD and a Crew Cab, so I would expect it to be on the low side. Plan on a weight distributing hitch and carry as much as possible in the TT instead of the bed. Axle ratings and GVWR ratings are available on the door jamb sticker - post a picture of it and you will get lots of good advice.