Recreational vehicles are excluded by default
Umbrella policies list what they sit above. An ATV or side-by-side that is not named, and does not carry its own underlying liability policy, is frequently outside the umbrella entirely. Owners assume broad personal cover follows them onto the desert and it generally does not.
A passenger is the serious exposure
Most off-road injuries involve a passenger rather than a third party, often a friend or a child of a friend. That is a liability claim against the household with the vehicle, and it is exactly the size of claim an umbrella exists for.
Underlying cover has to exist first
An umbrella requires a qualifying policy beneath it for each exposure. For off-road vehicles that means an actual recreational vehicle policy, not an assumption that the homeowners form covers it once the vehicle leaves the property.