Hello Michael
How many hour's are on the Onan engine ? A high hour engine can have a worn oil Pump or weak sending unit. If it persists I would switch out the oil sending unit and if that does not take care of it I would suspect that the oil Pump may be weak. There is a Bolt in the top of the engine on the left side looking from the front it is to the rear area of the engine above the Oil filter adopter on the top of the engine block it will have I think a 9/16" head on it. That bolt when Tightened down pushes on the Oil Pump spring that provides oil pressure. make sure that bolt is tight. I have heard of some guy's buying a new bolt a little longer in length to provide more pressure to the Oil Pump pressure spring. It has fine thread's on it and will have a copper washer under the head of it. I hope this helps, Kurt