Two batteries and two alternators in such a short time means that the problem continues.
One area that has not been mentioned is the wiring as I recall the RV is older and that leads me to believe that in the past maybe a battery terminal might have had corrosion, has anyone checked the actual wires both negative and positive for corrosion down the cable, copper wires tend to turn to liquid and impede current flow, another point is any one checked the negative cable at the point where it attaches to the frame corrosion at point of contact is likely, then there is the ground connection between the frame and the engine it is a point of corrotion.
Last but not least if there is a separate voltage regulator, has it been checked at all, it seems to me that the mechanics that have "repaired" this unit do not know much about the workings of the electrical system in vehicles and are throwing parts in order to solve a problem, instead of checking everything and then rendering a diagnostic that is real and actually finding the cause of this problem.
navegator