It could also possibly be a bad connection somewhere in the main power wiring. Check and clean connections at battery, the battery ground to the chassis (at battery) and positive & negative at the panel. Did all 12 volt loads work fine at first?
There should be a circuit breaker (like in a TT) just downstream of the battery somewhere but am not sure where in a FW. It could be faulty or the connections loose or corroded.
As said, get a voltmeter and check the voltage at a few locations. You should determine if the charger is working or not. With battery disconnected, measure the output right at the panel with 120V on, then off. Do this at battery too. If the battery worked perfectly fine when first installed, then it drained down in just 3 days, it sounds like the charger isn't charging.
While checking the fuses, check the reverse polarity fuse also. If you inadvertently connected the battery in reverse polarity, that could have blown this fuse.
Gil & Deb & Dougal the Springer Spaniel
2014 KZ Spree 262RKS & Ford F250 supercab V10 4x4 LB