A CPA or tax attorney would be the best source for accurate input on whether an RV or Boat would qualify.
The IRS web site does address the Tax Credit and RVs
Based on the information shown on the IRS web site, the fact that a motor home, boat or RV has a bedroom has no bearing on eligibility, but whether or not the RV is fixed to land or is just personal property and therefore not eligible.
A portion of FAQs on the IRS website says:
Q. If a taxpayer purchases a mobile home (manufactured home) with land and qualifies for the credit, is the amount of the credit based on the combined cost of the home and land?
A. Yes. The first-time homebuyer credit is ten percent of the purchase price of a principal residence. The total purchase price (mobile home and land) is used to determine the amount of the first-time homebuyer credit.
Q. Is a taxpayer who purchases a mobile home and places the home on leased land eligible for the first-time homebuyer credit?
A. Yes. A mobile home may qualify as a principal residence and it is not necessary that the taxpayer own the land to qualify for the first-time homebuyer credit.
Q. Can a taxpayer who purchases a travel trailer qualify for the credit?
A. A travel trailer that is affixed to land may qualify as a principal residence.
Q. Can an individual who has lived in an RV qualify for the credit?
A. For purposes of the first-time homebuyer credit, an RV with a built-in motor is personal property that is not affixed to land and does not qualify as a principal residence. Accordingly, someone who has owned and lived in an RV within the past three years may still qualify as a first-time homebuyer.
Q. Can I apply for the credit if I bought a vacation home or rental property?
A. No. Vacation homes and rental property do not qualify for this credit.