If your truck is already DRW, there is no need for adapters to run 19.5 wheels.
Most step-van chassis are GM, with GM axles/wheels.
Most built in the last 20-years have 19.5 wheels.
The GM bolt-pattern is the same, BUT the center opening is about 1/4" smaller.
It is a simple task for any decent machinist to lathe-cut the GM openings to FORD dimensions.
Although the 19.5 tires are super heavy duty, compared to an LT 16" tire, there are a lot of ride quality and wear pattern compromises when changing from LT-16s to 19.5s.
Also, there is a big difference in un-sprung weight.
Every set of 19.5s I have seen on mid-duty trucks that are set up to run them are dipped and scalloped badly before they have 12,000-miles wear on them.
Although the tread might last 75,000 or more, by the time they have 30,000-miles on them, they are so wash-boarded and out of shape that they get scrapped.
The above is considering that these tires are on a mid-duty truck; the problem is amplified somewhat on a much lighter weight vehicle.
Using CENTRAMATICs will help to some extent, but will not completely cure the problem.
If one has a big old heavy gooseneck that is somewhat under-tired with 16-inch rubber, the usual factory option is 17.5s, which are rare as hen's teeth and very expensive once one is found.
Such a trailer can be changed over to 19.5 wheels and be put to good use in getting all the wear that is left on the 19.5 used tires that have become so badly dipped-out that they were removed from the truck.
On a trailer, so long as one keeps them inflated to maximum PSI, there is little danger in over-loading a 19.5.