A motorhome is a "vehicle", Donna. Surely you've heard of those...most/all require registration/ Title changes etc. when changing hands. Just like gen-you-wine "All Molded Fiberglass Trailers" ! Y'all up to speed now?
If your relative has already tried to change ownership and is being refused because the Title in hand isn't notarized, the best and perhaps only choice would be to use whatever Oklahoma's version is of the so-called "bonded Title".
Most states have some version of this, which imposes a sort of waiting period in case somebody else turns up with a claim to a vehicle. In this case, the only person that MIGHT turn up would be the previous owner, and since you're trying to track them down anyway, who cares?
Just make sure to hang on to the proofs of the sale to them.