I've had Interstate batteries and thought they
were fine.  And 3 ain't all that old if it was
a good one.  I'd look for a slow drain on the
system...Are those wires capped?  They could be
shorting against something.  Also, are the
terminals clean & tight?  If not go after them
with an old toothbrush & a little baking soda
and water (just don't get the water in the cells)

Mike Wright
Ackworth, Iowa
68 Camper; 79 Bus
wright@storm.simpson.edu