As a fellow owner of a 2015 33D, I understand why you want to get away from replacing 4 batteries. Whoever designed the battery placement should be forced to water them monthly and replace a set. I spent two days removing the batteries to install a watering system and refurbish the compartment behind the rear wheels with a partial sheet metal floor. I will never replace lead acid batteries in that forward compartment.
That said, two ordinary 12 volt batteries would be a significant downgrade in performance.So 4 more expensive gel or AGM batteries will be my SECOND choice when it comes time to replace.
my FIRST choice will be two 12 volt lithium batteries. Performance and endurance are amazing compared to ordinary batteries, and prices are dropping. Still an expensive way to go, but if planning to keep your coach for many years should be a good investment.