Sorry, I remembered house batteries and not chassis batteries when typing.
A couple of assumptions and a theory....
2 chassis batteries means we are dealing with a decent sized diesel, so odds are a diesel pusher. With that assumption, I going to assume the house batteries (2-4 of them) are the wet type deep cycle, and could even be 6 volt?
The chassis batteries were the sealed no maintenance that exploded , and I'm questing that these where not the type of chassis batteries that originally came with the RV.
So my theory is that his charger for the battery system is a single battery type of charger, it may be multistage and multiple battery, but only one kind of battery can be used for both house and chassis batteries.
The Chassis batteries blew because they did not match the charging system and where over charged.
There are a few solutions, if the owner wishes to use different types of batteries in the house and chassis banks, then he will need to get a charging system that can handle that. They are a little pricey, but usually are a significant upgrade over what comes standard on an RV. There are chargers available that have a multiple battery bank set up, and you let the charger know what type of batteries are on that bank and it charges them accordingly.
The other solution is for him to check and see what type of house batteries and charger he has and buy chassis batteries that are compatible.
I hope I helped and didn't leave you with more questions than answers...