The dealer is responsible for any promises the salesman made that were beyond what the factory provides.
Steps, awnings, etc. may be promised and then not delivered. Decks, skirting (underpinning) etc. are normally provided by the dealer or a contractor working for him. It is to your advantage to find out and record exactly who is doing the work and ask them who you should contact if there are problems. Write it all down!
Be self-defensive in this process, take notes and ask questions before signing off on any delivery checklist. If anything is missing, make sure you have a written acknowledgement of the defect before signing off on receipt of the house.