I've arrived at something like that with our vacuflush. The automatic side of it no longer works but the vacuum pump does, that is connected to a on/off switch. Use the loo, turn the vacuum pump on, build a vacuum, press the pedal down, contents of loo get sucked into the cassette, turn switch off. The stinky gasses are vented out of a skin fitting. The toilet has no permanent vacuum so the water in it does not drain away and the whole contraption is reduced to its simplest and therefore its most reliable state, like most things you can chuck half of it away and it will work better. Oh, you can tell when the cassette is getting full as the vac. pump only runs for about 10 secs so you do need that microswitch.