The problem with most RV's is that they do not have enough cargo capacity once the fresh water tank and fuel are full.
I would still pick the E-450 to get the additional cargo rating. It would mean if you upgrade to a 5,500 pound cargo trailer with a 600 - 750 pound hitch weight will be no problem.
I have a 97 Bounder with a 2,734 pound cargo rating, weight of the RV without options, water or fuel. I have checked it a couple of times while on a long trip, and have been very close to the GVWR. But that is me. I have several options, and even leveling jacks add about 300 pounds to the RV base weight.
Fred.