Unfortunately you're not supposed to charge an agm battery with a lead acid charger. That will reduce battery life.? Look up "charging agm with lead acid charger" for comments.. The problem is a lead acid charger will attempt to push more charge into the battery as it nears full, whereas an agm charger will quit.
Check if your converter has a mode setting, and set it to agm if it does, and then you're OK.? You could also try and return or exchange the agm, but I wouldn't be surprised if they won't do that.??
If you're stuck with the agm and a lead acid only charger, don't leave your trailer plugged in for long periods, and that will reduce the problem, but you still won't get the expected lifespan out of the battery.
The situation you're in sucks.? You can get just about any solar charge controller, for as little as $15, and it will have settings for lead acid, agm, gel, and lithium.? But just about all the converters in our trailers, which cost $150 to $250, only support one battery type.