Are you sure it's no
passengers under age 18 and not no drivers

I used to work for Enterprise and I don't recall that being a requirement. However, I do know that they weren't allowed to be rented as school or daycare transportation, so maybe it was a regulation but I didn't realize it because of how it is worded.
I see you're in MD, though, and honestly, there are certain locations which you cannot even take them into, due to liability and insurance reasons. I believe D.C. is one of them, so depending where you're headed, you should be sure to check into that. I don't know if that was just an Enterprise rule or is true to all rental companies. It really is going to depend on where you're taking it and what you're using it for. You can try Budget, or check around you for smaller or less known rental companies.
Unfortunately, with this type of vehicle, it's almost considered a bus, so there are stricter requirements when renting them. It's been several years since I worked there and rules may have changed, but chances are you may have a difficult time with this type of rental without paying a lot in extra insurance
