The day after the sale is not the time to start questioning what the trailer comes with. You needed to do that BEFORE making the deal. In addition, when the dealer "said no", you needed to determine if what you weren't getting was a deal breaker.
Now that it is yours, you need to find ways to make it work. How about making a fabric cover for the sofa? Something that you can easily take off when you want to fold it out into a bed, but that will give you a bit more "traction" when you're using it as a sofa.
For the dinette, you're looking at paying the $500 for the remodel OR doing it yourself. You might get lucky and find someone who has a dinette and is willing to trade you the dinette parts for your chairs and table - then it's only the labor you're paying for (or doing yourself).