Here is my understanding of RV batteries.
Deep cycle batteries are more tolerant of deep discharge than other types of lead acid batteries. There is nothing magic about discharging to 50% or 6o%. The deeper you discharge it the sooner it will fail. 50% is usually quoted as a good compromise. Marine batteries are a compromise between deep cycle and starting batteries, not as tolerant of deep discharge but more capable of high current drain.
The 12 volt side of life is a good reference for learning more about the 12v system in RVs.
Most knowledgeable RVers on this forum who do much off grid camping prefer to have a battery bank of more than 200 ah, some much more. If you don't camp off the grid, any battery will do.
Re your genny, I would check the spark plug first, then see if you are getting a good spark.