For FIP validity I would strongly recommend that you never, ever take what a member of ground-level staff says as being gospel for anything other than "I'll accept it on my own train". Most staff don't actually know what FIP is, and tend to assume the validity is the same as for InterRail, which it most definitely isn't.
As examples from my own experiences in Germany, I've been told FIP coupons are valid on private operator trains (absolutely not), FIP coupons are valid in 1st class ticket despite being clearly marked 2nd class and (like almost all users of FIP in Germany) that the two day per box validity clearly printed on the ticket is flat wrong and it "must be" one day per box.
That last issue once got me a friendly chat with a DB Regio plain clothes RPI on board a train [heading from somewhere Dortmund way towards Hesse] about how useless DB Fernverkehr ticket training is, with the RPI being extremely blunt about how many problems he had to deal with because of incorrect information given to passengers by, and/or ticketing errors by, DB-F on-train staff.
I am certainly getting fed up with having to explain the whole concept of FIP to DB-F on-train staff, something I've not needed to do on DB-R trains which certainly suggests that DB-R training is better in that respect.
I'm surprised that RMV even know what FIP is, given that it's not something they would have any need to know about. But they may well have inherited someone from DB Regio, or they may well be rather more competent than is generally expected of German bureaucracy.
RDG Rail Staff Travel here in the UK quite often don't seem to understand the difference between "DB" (the national 'holding company', for the want of a better term) and "DB Fernverkehr" (the long-distance business unit operating IC and ICE trains commercially). It's ironic that the organisation representing 25-odd different train operators sometimes seems to struggle with the idea of another country having more than one operator!