Hi there. It is an old post, but hopefully, my answer will help other RVers with the same question.
Here is the answer: Generally (not always), the IRS considers RV a lodging unit. With lodging units, you are not allowed to take Section 179, unless you use your RV on a transient basis.
Basically, "transient" means that you use your RV as a hotel, where your stay is less than 30 days. If you drive around the country ( for business purposes only), you are not staying at one place for longer than 30 days. OR you are renting your RV to other people on short-term basis and never use it personally. In these two cases, you are allowed to take section179. However, even one day of personal use can kick you out of the transient status and that will disallow your Section 179 deduction.
If you can prove to the IRS that your RV is a vehicle, then you can take the whole 179 dedction.
How to prove? it all comes down to how you use your RV and good accounting records.
Source: I am a CPA and an Enrolled Agent