Derek you right the pumps do need repairs from time to time but the question is how frequently. I can see if the pump was running daily such as the one you use in your biz but most RV water pumps get much less use. Mine hasnt had a failure (touch wood) in six years camping 100 to 130 days a year - of which about 50% is using water via the pump rather than city water.
Second point to ponder is just how often would the situation arise that you need to repair the pump but cant empty out the tank.... the standard Scamp
tanks have drain valves attached to the tank right beside where the pump is and the water drains to the outside down through the connection in the floor. I suppose to keep down the number of valve connections/leak points one could replace that valve with a three way one if there is such a thing