Knowing our luck, the red light comes on seconds before you're hoofing...
Strange about the 40A fuse. The only 2 things that I know that could make it blow are 1) connecting the battery backwards and 2) The RR is overcharging the battery
#2 could happen if someone had extra stuff wired, like aux lights, and maybe an electric jacket. Or if it's putting out too much voltage. Check the DC Voltage at a high idle, say 2500 or 3500 rpm. The most you should see at the battery is about 14.5V. If it's 15V, it's too high, and you need to check for good solid ground connections for the RR.
#3 might be if there's a fault in the charging circuit (wiring) between the RR and the battery.