I figured out what you meant right after I posted.
But specifically, I see a parallel on hotels that offer day use passes to the storage facility that also has a Uhaul operation. The complimentary nature of a storage facility and a truck rental operation is almost the same as the complimentary nature of a hotel with a day spa operation. Some say NOT to include the Uhaul income. Why? The complimentary real estate uses can command higher revenues. The self storage doesn't need any significant buildings and the hotel doesn't either.
Not a self-storage guy or a hotel guy, but I do play devil's advocate -
While any income realized from leasing UHaul trucks would seem to be entirely attributable to the business, if I store UHaul trucks on-site, am I foregoing income that could be realized from leasing outside storage space, thus effectively reducing potential rental income attributable to the real property?
Again, not a self-storage guy, but from what I've seen, many have space for outside storage of vehicles (including RVs) and boats. That may be more common in areas like South Florida.