Kilr,
I have had long tubes on since Feb and just got off a 5900 mile trip with no problems at all.
If you are reading 11.8V with a new battery you most probably have a problem with the alternator. When the engine is running, you should be showing between 13.8V and 14.2V. I am pretty sure the voltage regulator is in the alternator. Remove the Neg batt lead and charge the battery up for about 6 hrs with a remote batt charger then re-connect the neg lead and try the starter with a fully charged battery.
My bet is the voltage regulator is gone.
DD