For purposes of the home mortgage deduction a properly-equipped sailboat or RV can qualify as a "home," so it seems logical that the same would apply in this case. My guess is that you won't get an absolutely definitive answer on this, however, until someone tries it and either the IRS accepts it or it goes to trial.
Remember, however, that the credit is actually more like an interest-free loan--you have to pay it back over the course of 15 years. Also, it cannot be used for a second home, a rental property, or anything other than your primary residence. Finally, to qualify, you have to meet certain income limits and you cannot have had any ownership interest in any home within the last three years.