truck is dead-after a jump start,it will run,but as soon as I turn it off-nothin

Have the battery tested.

alternator may be bad. the battery is running below 12 volts and all is turning off from lack of voltage. Roy

Alternator is probably bad. Also, check the fusable link between battery and alternator. To test, with car turned off, use voltmeter on battery to get a read. Then touch black to battery and red from voltmeter to wire on the alternator, the reading should be about the same. If read is 0 then fusable link is burnt out.