Gary, I can say that, in general, the combination of AGM batteries and solar charging can be a good combination. As with everything else, however, the particulars of the setup matter.
When I ordered my 2019 2552, I specified the AGM battery option and the 300 watt factory installed solar panel system. That combination has proven to be a good decision. One major advantage of the AGM batteries is that they are sealed and maintenance free--that eliminates the constant need to monitor and replenish water in the cells. I have only viewed my house batteries two or three times in the three and a half years since I got my coach in early 2019--just to make sure no corrosion or other issues was forming on the cables. They require zero attention. The factory installed solar charge system does an excellent job of keeping the house batteries fully charged as long as the roof-mounted panels have sunlight. Again, it requires no management--I just check the battery charge status indicator occasionally when I'm in the coach and the batteries are always at 100% charge. That setup is about as reliable and trouble free as one could ask for.
I believe that the only negative in changing your house batteries over to AGM is the cost difference from the flooded cell batteries. Beyond that it should be a direct change-out of taking the old batteries out and putting the new ones in. I'm not sure what brand AGMs Phoenix is installing in new coaches, but the Trojan T105-AGM units they were using in 2019 are excellent batteries, and I wouldn't hesitate to purchase them again. Of course a heavy move is being made to lithium battery technology in the RV industry today (Phoenix included), but for many RV'ers lead-acid AGMs meet all the need at a lesser cost.
Regarding the solar battery tenders--I have seen the units and the concept is good, but I don't know how they perform in real life since I've never needed to use one. As I said, the roof-mounted panel system installed by Phoenix has been flawless in my experience, but that is a quality professional installation using good components. It does seem feasible to me to purchase a quality solar tender, place it where it will get reliable daily sunlight, and route the cables to the batteries. Another option would be to start and run the generator every month or two during your six-month storage period; running for an hour or so should replenish the batteries as well as provide the side benefit of preventing the generator carburetor from getting "gunked up" by stagnant gasoline.
All the best, Mike
