First of all
A few things to look for:
Did you charge the battery? I realize you say its fully charged, but wondering if you fully charged it with your own charger. New batteries might not be fully charged unless someone did this. Usually the place you got it from does this for you, but some just take if off the shelf where its been sitting for awhile and say its fully charged.
Confirm that you have it wired correctly with the negative cable grounded to the frame and positive leading to your converter. Miswiring is an issue we have seen before.
Confirm that your fuses and breakers are good. There should be an inline fuse on the positive cable running to the converter close to the battery. This could need replacing or it might be a resettable fuse. This is a box that has a tiny button you can use to reset it.
That's a start and let us know how it goes.