The solar array size is fine, but with solar bigger is better. The batteries, on the other hand, are pretty minimal unless all he is going to run is lights. As mentioned above, he needs to work out his expected loads (total watt-hours) for 24 hours and size his battery bank accordingly. He also needs to think about how he is going to use the RV. If all he plans to do is stay in sites with electric most of the time and boondock for a night now and then, the 200 amp hours of batteries will be an OK minimum. However, even that depends on what he expects to power. Running coffee maker, microwave, hair dryer, TV and sound system will flat 200 amp hours of any kind of batteries in short order. He also need to be aware of hidden loads like the electrical system on the propane fridge and the propane solenoid valves in the fridge, water heater and space heater. Those things alone can easily use 30-50 amp-hours per day. In contrast, if he wants to boondock for multiple days or even a week at a time, he needs to size his battery bank to handle all loads for 2-3 days at a minimum because it isn'ty always sunny and RVs sometimes get parked in full or partial shade.
As far as the number of cycles to expect from the batteries goes, the deeper the discharge between charges the fewer cycles you will get from a battery. That applies to ALL types of batteries including lithium. So if he under sizes his battery bank he will kill the batteries quickly. In contrast, over sizing the battery bank will give much longer life. Now, as to battery types. Gels would be my last choice because they are the most expensive of the conventional battery types (Flooded, AGM, Gel) and don't give enough extra bang for the buck to make up for the cost difference. As stated above, Gels are also more fussy about charging voltages than other batteries and the converter in the RV (DC power supply/battery charger) is highly unlikely to treat Gels well. The solar charge controller won't be a problem IF he gets one that has a Gel setting or is programmable (Victron). Battery type depends on a number of things including budget, concern for battery off gassing and how long he expects to own the RV. Budget is generally the biggest factor. As far as budget goes the order of preference is 12V Grp 27 or Grp 31 flooded deep cycle, flooded 6V golf carts, 12V AGM, 6V AGM golf carts, lithium iron phosphate. If your son is concerned about battery out gassing then he needs to go AGM or lithium. Finally, if he intendsto own the RV for 10+ years and has the money it is worth considering lithium iron phosphate batteries such as Battleborn batteries.
My suggestion for the short term would be to go with four 100 amp-hr flooded 12V deep cycle lead acid batteries. He can get decent batteries for $100 each. Unless he charges them at very high charge rates off gassing won't be a big problem, but they do need to be in a vented location. I can't say more than that without know what his expected loads will be (i.e., lifestyle and RV size).
