Hey Robert!
Yes, my partners & I are currently involved with one in Punta Gorda, FL.
Boat & RV Storage is indeed a niche strategy. You'll typically see them partnered with traditional Self Storage - meaning those Facilities offering Storage Units for folks to store their household treasures AND on-site parking spaces for Boats, RV's, company trucks, etc. That way you kind of hedge your bets (aka Risk/Reward).
Self Storage is historically Recession-Resistant as it has everything to do with Death, Divorce, Down-sizing, Dislocation, etc. - things that are prevalent in normal times and certainly, in not-so-normal-times, too.
However, RV & Boat Storage is similar but different, in that approx 1 out of 10 folks in the USA rents a Self Storage Unit, and that is not the case with Boats & RV's; hence it being a much more niche strategy. You don't have the general population to fall back on for your historical & future trends.
Also, Self Storage is needed in good times & bad, but that is not always the case with Boat & RV's. When a large corporation is in financial dire straits, one of the first things they get rid of is the company jet. Same can be said for a household. The boat & RV is usually the first to go.
But we are certainly in interesting times right now as RV's & Campers seem to be in hot demand with some places having a hard time keeping them in stock and on the lot if it's a good deal. They're being bought quickly. And unless they're strictly for living in full time & travelling, they're going to need to be stored somewhere, right? What I'm saying is you can't necessarily use conventional wisdom for 2020. Question remains is: what does it look like in the years post-2020?
However, if you do your market research to determine the supply & demand and look at what other Boat & RV Storage Facility's are doing nearby, and pick a good market (Florida - the entire state is practically a boaters paradise, and for those that don't boat, well then, they RV. Granted that's a broad generalization, but I think you get the point.) You have to pick & choose your markets just like you have to pick & choose your assets classes.
And yes, the SBA does allow for Boat & RV Facility loans. Course, I'm sure you know, the SBA doesn't actually lend any funds towards your business, they just underwrite & (partially) guarantee the actual lender who is loaning your business the monies. So you'd be subject to that banks underwriting (in general & SBA) criteria, as well as their primary focus on businesses. (Meaning some banks may, some may not - you can't force them to loan you the funds just because they are an SBA-approved lender.) But if you have a solid plan, most likely a feasibility study, you ought to succeed in finding the funds. Course that means your plan makes sense with or without government money.
I hope this helps.
If you have any specific questions - please feel free to post here and/or PM me.
Good luck to you!
Scott