Have not done similar. But, read the voltage inputs range for the maintainer, if the 7V drop is still in that range, you should be OK, if not, got to 14 ga which should make a huge difference. In either case, use high quality cord ends or you will lose more voltage. My guess (only a guess) is it will be OK, but check the specs to verify. Then run it, and get an in use voltage reading. For the test, have the pump moving water so it is under a load. Then see if the voltage stays good when the pump starts, or at least only a momentary surge drop in voltage.
Dave Klish, I recently bought a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.