Please note that I am making these suggestions to help you enjoy your new camper as much as possible. I say that because if you take back to your dealer chances are they'll have it for weeks to months waiting for parts. Here are my suggestions for each of your concerns:
Replace with 15" rims/wheels - Have Forest River ship the new wheels and tires to you directly and you put them on. If FR wants the old ones back have them provide you with a UPS return tag.
9.5 gallons before overflowing, they have vent pipe stuck too far into tank creating an air pocket in the tank - Have your dealer pull the grey and black vent pipes up higher and cement in proper place.
Practically all the pipes water/gas/drain are NOT sealed to the outside allowing road water into the insulation - Not quite sure where you are talking about sealing but I'm going to assume under your trailer. I would seal them my self after all dealer & factory repairs are done. Use some black (pond/water feature) expanding foam spray and you're good to go.
Fresh water tank said 21 gallon capacity , but is only 15 gallons..they say you have to count the 6 gallon water heater capacity?? - Here is where the devil is in the details. Most specs will say "fresh water capacity" but if yours says fresh water tank capacity then you are right and the factory is wrong and then they'll say "specs can change without notice" Nothing to fix here, sorry.
Fresh water tank loosely connected to floor, can move 3 inches either way!! - It is designed to move because the tank will swell in size when filled to capacity. Only worry I would have would be any fittings that could be sheared off because of the movement.
Hot water heater never shuts off..have to turn pilot off/on. - Are you sure on this one? If it never shuts off the pressure relief valve would pop and spray water onto the ground. Has that happened?
Scraps of wood sawdust under storage space under bed. - Poor dealer cleanup and prep. Pull out some of the drawers and you'll be surprised at what you'll find.
Toilet defective and leaked into back inner chamber!!! - Make and model of toilet will help with this one.
Stabilizer jacks each had a broken bolt on when purchased - Buy a replacement bolt and fix it.
Gas lines were danglining underneath unit by 6"..dangerous. - When you are at the hardware store getting a replacement stabilizer bolt pick up a couple of large nylon wire ties and use them to strain relief the gas lines.
