You need a charger that will stay on the job for a few days, not an "automatic" that shuts itself off after a few hours. Your trailer's converter is one. Leave it on that charger for a few days and then check SG again.
You could really use a "manual charger" that gets the battery up to 15v for an "overcharge" but you need to unplug it yourself when it gets the battery voltage up there. Then leave it on the converter for a few days and then check the SG.
However if you have only those two chargers, try it with converter first before rushing out to buy anything.
