I dropped by a local U-Haul dealer, and had a look at their trailers. Between two or three sizes of open cargo trailers and several sizes of enclosed cargo trailers there were at least five different bolt patterns
. Of the 5-bolt 14" setups, there were two or three combinations of bolt pattern and offset. One of them looked to be 5-on-4.5", with a conventional offset, but another had a significant positive offset (like a typical front-wheel-drive car) and a bolt circle diameter which might have been 4.75", but might have been between 4.5 and 4.75". Maybe that's the oddball one used for the trailers? The ones I saw had "U-HAUL" stamped right into them, and the tires
(ST205/75D14 by Goodyear) had "Property of U-Haul" moulded into the sidewall.
I was measuring with a copy of this template, printed on clear plastic stock:
With only the domed ends of the wheel nuts to go by, it was hard to be accurate, and I had forgotton to cut out the centre hole so trailers with protruding hubs were even worse.
While I was looking (after the place had closed, leaving their trailers accessible in an open parking lot) an employee came by to ask what I was doing. I told him I was looking a cargo trailers for some stuff I have to move (which is true) and that some people I knew of had U-Haul travel trailers and were wondering about the wheels.
He had heard of the travel trailers (which he described as little white fiberglass trailers, "for a Corvette"), and knew that the wheels were an uncommon pattern. He said they were not unique to U-Haul, and thought they were some metric size
Well, I don't believe this guy is any kind of expert, but perhaps - given the automotive style offset - that one type is something like 5-on-120mm, to fit some sort of car? I don't even know if that's the same wheel the travel trailers use, but it's an interesting possibility.
A random wheel supplier I checked shows wheels with many bolt patterns, including 5-bolt patterns in the following bolt circle sizes in this general range:
112 (4.409" or 4-13/32")
114 (4.488", maybe just 4.5" rounded to the nearest mm)
120 (4.724", listed separately from 120.65mm)
The ones which list a 5x120 mm bolt pattern come in various offsets from 3 to 42 mm, and since at least one brand showed 120 mm and 120.65 as distinct sizes, I Googled for "5x120 wheel", and learned that this is (among other things) a BMW size, in 20 mm or higher offset (e.g. Discounted Wheel Warehouse; BMW - Vehicle Bolt Pattern Reference
I also ran across a GM pattern listed as 5x115 at Discounted Wheel Warehouse; could be a typo (likely for 5x114.3 a.k.a. 5x4.5"), or it could be another oddball size.
So in the end, I don't have the answer, but maybe some possibilities to check out. If U-Haul really used a unique bolt pattern, or even just one for which wheels exist but trailer hubs usually do not, then I guess that's a testament to the extent to which trailers can be customized! They certainly don't put unique bolt patterns on their trucks.