Just like in house wiring, the neutral is always white. Trailers I have experience with both had the 12VDC positive black (like black = hot in household single phase 15A circuits). Not to say you couldn't have red = +ve/hot too (corresponding to the hot on the other side of a split phase household circuit).
The only place I see black=ground/red=hot is in pure DC applications (e.g. automotive).
With trailers having both 12VDC and 120VAC wiring, its safer to have a consistent white = neutral/ground standard - in fact white/grey = neutral is mandatory in both the US and Canadian electrical codes for both AC and DC (Wiring Color Codes : Color Codes
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