You do not want to use romex or any solid wire in a vehicle. The flexing and vibration will eventually break the wire as it is not designed to be flexed.
THHN is industrial wire. If it is solid wire, don't use it for a vehicle or ATV, same reason. Stranded version would be ok but it is also not rated for it. (it won't be flexing that much in a vehicle)
MTW wire would be the best bet, but for what your doing if the THHN is stranded, go with it.
A lot of charts show voltage drop over length but that is for AC voltage. DC voltage has a higher drop rate over distance.
My general rule is I bump up one gauge for what is needed.