New Battery or Battery Charger

Lights don't work unless I have it plugged into the truck. Battery switch is always on in my case. Feel like I may be doing something wrong. What would you suggest I do? My next step was to charge the battery as it's only registering about 6 1/2 on the multimeter.

Converter should put about 13.5 volts on the battery continuously.
Did you turn the battery switch off? Do the lights work?

As noted earlier, you need to get a multi-meter and test at the battery. You should while plugged in, see a healthy 13 volts. Disconnected, your battery level should show. Should be around 12.5 volts.
No 13+ volts then the converter or converter circuit may have issues.

Had exactly the same thing happen. Measure the voltage at the converter terminals themselves, then at the battery terminals. It should be the same, over 13 volts.

If it's not, then there is a loose main feed from the converter to the battery. In my case I had to tighten the screw holding the negative wire right on the negative terminal strip.