In our 5er, the TP holder didn't come installed either. The only accessibly wall is curved so can't really mount to that.
However, the vanity is pretty much straight across and in easy reach of the commode. So I glued a piece of 1x3 pine to the inside face of the vanity door (Gorilla glue and c-clamps) to provide depth for the screws and mounted it to that. Takes up almost no storage space and out of sight until needed!
This should really go in the 'Most Useful Mod' thread (although it wasn't really a 'mod').
Pix: closed and open, from the commode
2015 Rockwood Signature UltraLite 8282WS Platinum, GY Marathon LRD, TST 507RV TPMS
2005 GMC 2500HD CCSB D/A, Curt E16, Prodigy P2, Garmin RV760LMT w/BC-20 b/u cam