I would be worried that an emergency inline pump, when not being used, would create some restriction that could accelerate the demise of the in-tank pump.
"If" you can draw fuel through a failed in-tank pump, then the easiest way would be to install 2 T's in the main line, with a shutoff valve between them on the mainline. Plumb the new pump between the T's in a bypass line. While the in-tank pump is working, the valve is open, allowing fuel to flow to the engine. To use the backup pump, simply close the valve and power up the pump.
If you can't draw fuel through a failed in-tank pump (as your RV repair place claimed), then the generator pickup is all you've got to go with. The pickup tubes on my last RV's in-tank pump were all the same size, not sure if that's the case with yours.