Understood, Mark. My point was that it depends on how much the van weighs. I would be exceeding the 20k rated capacity of my buses to tow 9000lbs. They weigh a little over 11,000 AFTER I stripped the inside of everything to haul cargo. With the cargo, or even the 20 seats I pulled out of it, it would be well over 20,000lbs if I pulled that trailer.
I haven't weighed the ambulances, but they're prolly close to 10k, which means I'd be pushing the limit of those too with that trailer.
I'm not sure what the 450 RVs weigh, but I'd be surprised if they were less than about 14k.
The OP didn't say what kind of 450 he has, but none of them are the base weight (5-6000lbs). They only come as cutaways or stripped chassis that I've seen, which means they all have some kind of aftermarket body on the back adding weight. That's exactly why Ford doesn't list a towing capacity for them.
Maybe it's the gearing in my buses, IDK. But I've pulled trailers with them coast to coast, north and south, prolly close to 100,000 miles, over the mountains too many times to count, and all I know is that when I get much more than 16k total weight I literally have to crawl up mountains going 25-30mph or my temps will blow. I've had to pull over more than once to let the engine cool down from +230. Even driving flat interstate at that weight during the summertime, my truck's ECT/EOT pretty much just cycles continuously - heating up over 224, fan kicks off, runs until it cools down, then repeats, over and over. Winter, of course, is a lot better.
But, I do expect that I've had as much as 17k total weight at times - prolly those times I had to pull over.
But, I could see how a much lighter van, like a SRW or maybe a DRW with a short box on the back, could do it. Honestly, I expect the only way to reach the 450's 20,000lb combined GVW limit is with a heavy RV pulling a light trailer.
But, this is an old thread that I happened to see - I haven't frequented this site much - and didn't want others who might stumble on it to not consider the weight of the van when calculating towing capacity.
Maybe the OP will come back and tell us what type of body his van has and how it fared pulling his trailer. I'd be interested to hear.