Rather surprised you haven't had a response on this. I doubt if you NEED a sign-off, after all you are flying within the privileges of your license, which is an international license.
On the other hand your insurer may require evidence of your actual tailwheel experience, including on type, and that might require some tailwheel time on type.
In any case, unless you have experience on type, I'd say it would be wise to get some before you take any risks in your lovely RV! Tailwheel RVs aren't hard but there are some tips and tricks, as with any type.