When the IRS has taken people to court for this, RV vehicle used as a home office has not been allowed by the IRS.

Can an RV Qualify as a Home Office? | Nolo

Thanks George!  Would the mortgage interest still be deductible?  My earlier research seems to indicate it would be.

I have posted the reason the taxpayers didn't qualify in their situation. your situation could be different.

To qualify for a home office deduction, a business owner must use a portion of a dwelling unit regularly and exclusively for business purposes. The tax court found that this couple had failed to prove that there was an identifiable portion of their RV that was used exclusively for business purposes. The area they claimed constituted the home office was the countertop that the husband used as a desk. But the court said that it was simply not believable that "in the cramped quarters of a motor home, an unclosed area like the countertop would somehow be exclusively reserved to business activity." (Dunford v. Comm'r, T.C. Memo 2013-189.)

For the interest to be deductible on schedule A as home mortgage interest the RV must qualify as your primary or secondary residence, To qualify it must contain sleeping quarters, a kitchen, and a fully functioning toilet.