Unfortunately... there normally isn't a good fix. Basically, the center crank seals gave out. That allows the oil to flow from the oil tank, and into the engine.
If you want to take the time... you could split the engine, and replace the crank, and RV seals. But generally... it's best to just replace it with a rebuilt engine.
With that said... you can pinch off the RV hoses, and get the engine cleared. If it's been sitting for a long time, and this happened... then you may still have a little life in it. BUT... if it re-fills in a few days to a week... then it's just too far gone.