I'm 400lbs and I'm getting 25-30kms but I'm wired up to 2 battery packs (about 24Ahrs)

check you tire pressure. are you running knobbies?

If you have SLA batteries the voltage will sag after you have ridden for 10 minutes or so. Are you getting dead batteries at 5 miles, or are they just not as strong as the first 5 miles?

My batteries sag fairly quickly, but then they continue to work for quite some time after.