I'm looking for a RV transporter for heavy 5th wheel

X2 It sounds like you put the cart before the horse. 20,000 lb will be as shipped from the factory, it will go up exponentially as you add propane, water, bedding, food etc. IMHO you are wise not wanting to chance it with your current truck, wind across the plains could make it a wild ride and steep grades will give you grey hairs and religion.