The only gasser class b that is still, barely, in production is the Chevy Express based versions at Roadtrek or a custom builder. They have a very reliability history. The no longer produced Dodges and Fords also were reliable.
Currently, you would be looking at the Promaster Dodge or Ford Transit, both of which have have engines that have been around quite a while and been quite good in other applications. Neither have a long history of dragging around a fully loaded class b yet. The Ford engines have been used in a lot of pickups, so that helps, but pickup drivers who haul a lot get diesels most of the time, so may not apply. The Dodge history is mostly in minivans with the same front drive configuration as the Promaster. Both have done well so far. My guess would be the weakest part might be the Promaster's transmission and front drive components as, AFAIK, they have never used that setup to pull that much weight, all the time. But that is just a guess. If I had a choice, I would likely chose the Ford based on their truck experience history.