I would put some vegetable oil in the toilet and the sink. This is what I do with my hunting 5th wheel. It sets unused for months sometimes, so I drain the water out of the toilet bowl and put an inch to inch and a half of vegetable oil in it (help lube the seals too). Oil floats on water and won't evaporate so a half cup in your sink would form a barrier over any water in the trap and keep it from evaporating. The vegetable is environmentally friendly and won't hurt the pipes or tank. I also crack a vent and a couple windows for ventilation.
