Every land based sink has a water
trap in the drain that is there to ensure that sewer oders do NOT come back up the drain, so the low spot you would have would not be a problem for that reason. This arrangement would only work if the head sink stayed above the waterline all the time.
In many older cruising boats with compact heads the sink drained directly into the toilet and was pumped out from there, that would not be a solution unique to you.
If you do end up with a sump&pump arrangement, the sump can be as small as possible and still fit a level switch in.