2013 Devon

Am new to motorhome so would be grateful for advice. on the Sargent 480 it appears I am using between 2 and 5amp from my battery when the van is in my driveway. After approx 5 or 6 days I get a warning on the 480 that my vehicle battery is running low, have I a problem with electrics? or do I require larger solar panel. (40w at present )Did try to go off grid once but found low vehicle battery warning was given before the end of first day.