Your suppose to some water in the toilet bowl at all times - helps keep the gasket from drying out and controls odors. Many will keep a small amt of water in the black and gray water tanks between camping trips to help keep drain valve gaskets from drying out. During long term storage use antifreeze in toilet bowl - some will completely empty tanks and leave valves open - some like me will keep a small amt of antifreeze sometimes combined with a lubricating oil to keep gaskets from drying out.
Can you pee in the toilet when stored - I do on occasion and don't see a reason you can't so long as valves are closed and you make sure you add water/antifreeze to keep some liquid in the toilet bowl.
Not sure where the insects came from - hatch of some kind. Keep some water in the toilet and valves closed and I probably won't get a repeat.