1. Sewer Hookup. When the trailer has holding tanks, you need to let the quantity build up some before emptying it. If the tank is constantly drained of liquids (i. e. leaving the dump valve open) the solids will collect on the bottom of the tank and never get emptied. It's best to let the holding tanks, especially the black water tank, get to at least 2/3s full before emptying.
Since an RV toilet has no p-trap, there is also the chance of getting sewer gasses from the sewer system back into the holding tank and maybe into the trailer.
2. Many people, myself included, use some kind of chemical additive to help keep odors down in the black tank. There are some that can go in the grey water tank as well. If all is plumbed correctly, vented correctly, emptied frequently and flushed well this may not be necessary.
3. Can't help you here.
Good luck and happy camping!