Many many RVs are used exclusively for full hookup camping and the battery is just serving as enroute power. As Tatest points out many RVs are used only occasionally and sit for months in storage lots. Not really a good market for high tech high cost batteries.
Someone like myself would appreciate them if they were a reliable and somewhat reasonably priced package. Downside would be our charging systems would have to be upgraded.
I use a couple AGM batteries rated at 300 AH. They work fine, the space is allotted. My goal, rather than increasing capacity is to cut demand. With newer LED TVs, computer screens, LED lighting, and the great reduction of solar cost we can take these amp hours much further. There are many very good inexpensive strategies to improve the 12 volt systems for people like myself who use it frequently.