I've found some more information on this and it seems that IRS is taking a different position for the First Time Homebuyer's Credit than I expected (remember, they don't write the laws, they just interpret them). According to the IRS, a boat or rv can be your personal residence for purposes of deducting mortgage interest, but not for claiming the First Time Homebuyer's Credit. Some Q&A posted to the IRS web site:
"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."
I'm not sure if this would hold up in court, but I would not recommend fighting them on this one.