You can measure the parasitic drains my removing your neg battery cable in setting your multimeter to the 10 amp scale and measure between the neg cable and the neg post. If you measure an amp or more that would draw your battery down. You can always just disconnect your neg cable for long term storage if you have two much draw. I do this over the winter months and the batteries are down maybe 40%. What you describe is not right, either batteries were not fully charged to begin with or something is drawing from you batteries.