Classic.
"Best way" is 3 stage charging at correct voltages to full 100% with temperature compensation. Without solar as the finishing stage you won't get 100% unless you run generator all day long. You don't have solar and don't have grid power to run some good 3-stage charger day and night, so forget about best :).
But it can be improved a little.
If your converter is not 3 stage with adjustable setpoints and temp comp, then you need a better converter or standalone 120V charger.
If voltages at Bulk and Float - measured on converter output - are what they should be according to battery specs, but on battery terminals they are lower than should be - then you need a shorter or thicker DC cable to battery. If DC cable inside the trailer is not good enough, it will be a pain to replace it. Will be easier getting a good 3-stage charger and hook it up to battery with a short cable.
