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

I am new to RV'ing, and looking to purchase a used rv to pull with my GMC Sierra 1500 Crew Cab 4WD gas burner w/5.3 Litre engine. I have two daughters, age 11 & 12, so it will be for a family of 4. I'...