Like others advised, we need to know more about your equipment, and about how you use it.
But a couple of thoughts:
Most people kill their batteries by running them too low before charging. You should be charging every day you are camping. Not waiting until the batteries are depleted. Likewise, unless you have power connected in your storage area, you should not leave the batteries connected during storage. You may ruin them in a matter of days from parasitic loads.
Those "Deep Cycle Marine" batteries, probably are not. They are combo starting/house batteries. I can say that with relative confidence, because unless you really search, that is all you will find in the usual places.
Most trailers built in the last 10 years have a decent multi-stage converter, and I don't know what you have, but sitting here I doubt that the problem is with your converter. Much more likely that the problem is in how you use it.