The weak link of the E-450 is the stock tire load range. The axles can handle more but the tires are the real issue.... especially if under-inflated, over-heated, etc.
Do yourself a favor and put a set of Commercial Rated 16" tires on the rig like these:
https://www.goodyear.com/en-US/tires...tude-ht-c-type
Legally you will still be over the GVWR, but the tires will be able to handle over 18,000lbs and the likelihood of a tire failure will be greatly reduced because they tires are meant to handle heavy loads all the time unlike an LT rated tire.
The garage in our Outlaw 29H said it was rated for 1500lbs. When we put our 1000lb Syder in the garage we were 1000lbs over GVWR. With nothing in the garage, basic gear and full fuelm water and propane we were pretty close to the max weight rating.