I had severe trouble finding a storage unit tall enough for mine too. 12’ doors were standard. I saw a new storage construction project going up and rushed over to tell the owner to make accommodations for the tall motorhomes as you mentioned. He did make some of his units with 14’ doors and 50’ lengths, which were filled promptly. He thanked me, but did not give me a break on rental fees.
Later, I built a proper wing on my barn with a dump station and 50 amp hookups.