There was a long thread on a fellow who got a back bill from the IRS for a few thousand dollars because his Class A wasn't considered a second home.
There is a form 1098 you get from the mortgage company listing the amount of interest to the penny. However almost no RV loan lender sends that form out to RV owners. Deducting RV interest - you need a statement or other proof of the amount of the deduction.
This fellow also owned some unimproved land - no house or other facilities. But the people holding that mortgage send out a 1098 on that loan.
You can see how the IRS would think the two 1098 forms (his home and the unimproved land) equals two homes - and disallowed the RV interest deduction.
It took a phone call and some work to get the issue resolved.
BTW - for some of us the RV is the first home. Don't have another one.