If your attorney who wrote the deal for you didn't cover (in the purchase and sale contract) belongings left on property after closing, then they didn't do their job. What's the contract say?
Sounds like you're being considerate and accommodating, which is the right thing to do, IMHO. If you were told (verbally) that you got to keep building materials, and there is a clause in the contract about personal property remaining after closing being yours, then you are fine. If your attorney didn't cover this scenario, then my suggestion would be to talk to them and suggest strongly that they need to advise you (for free) on what the law is regarding personal property left on site. Hopefully you will not have to incur significant cost to clean up or dispose of anything.
Things that are titled, such as the truck and motor home, would not normally be covered in a real estate contract. Each state will likely have different rules on titled items like vehicles. If nobody wants them or is willing to remove them, then you will likely have to file some sort of a report of abandoned vehicle report. Depending on your local/state rules, you may be able to petition for title and assume ownership if you follow all the rules to a "T."
Good luck, and congrats!