Can't see the photos... can you repost?
Don’t try starting the ski again until this is sorted out. If the return line is not kinked, and the check valve is functioning correctly, you have coagulation in the oil system (this can be expected with multiple oil types.)
If the system is coagulated, it needs to be flushed. I would also recommend removing the injection pump and cleaning it if coagulation has occurred.
It’s entirely possible that you have a pocket of air somewhere in the RV system. Remove some of the oil (to below the top of the valve) and check the check valve. If it’s working properly (air moves in one direction, into the tank) remove the top oil line (with the tank cap off) and blow air through the return line (no more than 5-7 PSI.) Be extremely careful, and whatever you do, DO NOT exceed 5-7 PSI unless you want to rebuild the motor.
If there’s an air pocket, it will clear almost immediately.
In either case, best to replace the oil lines, the fliter, and flush the RV cavity.