Biggest issue is the converter or inverter/charger how it charges the batteries. Too high of charge will boil the batteries causing sulfur smell.
Selection of batteries depends on your type of camping. One week once a year does not justify the following, switching to 6 volt true deep cycle like the golf cart batteries. Can get these for around $80 at Costco.
Minor dry camping you could get by with standard group 24 or 26 but need to be deep cycle.
My guess your converter is over charing the batteries and you are not keeping them full of water.
What is make and model of your converter?