The coach battery in your Fiver should be in good condition, and it should take a charge from your built-in converter.
One thing to check first is your CUT-OFF switch that is there to disconnect the internal 12-VDC equipment from the battery when the RV is put in storage.
If it is in the right position, then your converter should charge a good battery.
1- Check how old the battery is and is it too old to provide service anymore.
2- Check the battery's condition and if the battery has water in it.
3- Check if the battery is taking charge from the Converter.
FYI - If you use a multimeter to measure if your battery is charging check this; a battery that is being charged will read around 14,5-VDC, a fully charged battery will read around 13.5-VDC while an uncharged battery will read around 12.5-VDC.