Might help if you mentioned which one....
After all there's the rarely spawning Romero hearse (the Albany Washington/Lincoln Town Car based one), and the Halloween special one (i forget it's name, but it's based on an Albany Buccaneer in 2 dr SW/SB form).
*SW/SB - refers to Station Wagon (or Estate) bodied, SB as in 'Shooting Brake' which is essentially a short-bodied SW/Estate care often recognised as a short 3 dr estate similar in style to Volvo 480 and other similar sized, config cars - includes a few Honda 'Aerodeck' designs and at least one recent BMW).*
Not that i can help, but it helps to be specific if you expect people to try to help. Just be aware, that even with the presence of another example or being in one, these are still incredibly rarely spawning beasties (by normal spawn distribution probability) and unlike a lot of motors in GTA:O, the actual combinations of vehicles in proximity which can trigger a spawn is somewhat haphazard on a good day - and that's just the Romero, god only know about the Halloween special.
Most Romero examples you see as PV's (rarely, reflecting both obscurity and rarity) were either a) glitched in from SP pre to CT using an account previously on XBOX360/PS3 or via a Garage Editor or Vehicle spawner on one of those two systems and 'ported over' along with account/characters in a CT. There's bound to be the odd few natural spawners in NG GTA:O sessions, but that's because someone spent a ridiculous amount of time crawling the cemeteries and funeral homes in game with god knows how many variations of trigger vehicle. Technically, by association, some sedans (Albany Washington or a Stretch) should be the natural trigger vehicles (as the Romero should share some probability as well as basic design with these), but if you look at the vehicle meta on the old-gen copies of the game - the Romero is virtually a law unto itself over spawning and shares little probability traits for spawning with anything remotely similar.
Good luck, you'll need a crap load of it (kinda on the scale of a hurricane scale sh*t storm) getting one to spawn in NG, it was tough enough in the pre-gen versions and the probability distribution on the NG of different vehicles is widely different to the prev-gen.