Just noticed this thread. I posted about it a few years ago here https://pinside.com/pinball/forum/topic/how-many-pins-vids-fit-in-a-u-haul#post-5002795

But to provide the secret sauce, here's what I wrote

Using a 20’ u-haul, I was easily able to load 11 pins and 4 vids. This took up 90% of the floor space.
You should be able to put another 3 pins in there.
Generally, assuming vids and pins have similar moving dimensions, you should be able to fit 18 pins in a 20’ U-Haul’s available floor space.