Many of the small macerator/lift pumps used to get sewage into raised-bed drainage fields for single-family homes are quite small. Most are under half a horsepower. Combined with a small reserve storage capacity in the pump chamber and drain lines and it should be possible to run the pump off a simple inverter connected to a vehicle a couple times a day.

You could run the well the same way. Running the well off an inverter you could store water and get as much heavy use done as possible. Then switching over to the septic system you clear the lines.

It would take a bit of coordination and handiness with electrical equipment but it isn't that complicated. Easier still if when the well and septic pumps were installed the installation incorporated the extra equipment that makes external connections easy. Something to ask your contractor about.