The RJ85 can be fitted with 3 extra fuel options:-
Panier tanks, a
Rear Bay tank and a
Front Bay Tank. Each option increases range at the expence of payload.
There is a graph here,
2016-11-28 LAMIA Avro RJ-85 crashed near Medellin with 81 on board » JACDEC scroll down about 1/3 of the document.
We do not know which tank options, it any, were fitted or how much fuel was actually loaded. It is also unclear whether the quoted ranges are with or without reserves but no sane pilot would have departed without reserves for a diversion plus 60 minutes so we must assume that some extra tanks were fitted.
The fuel theory is most likely a total red herring, expecially in view of the reported electrical problems.
All speculation is futile until we have more facts.