Nissan: electrical problem?, long periods of time, nissan frontier

Question
QUESTION: I have a 98 Nissan Frontier 4X4, 120,000 miles, manual transmission.  I replaced the battery 1 year ago.  The truck seemed to be getting slower to start over a couple of weeks then I went on vacation and the truck sat for 1 week.  When I got back, the truck started just fine but when I drove it for the first time, it ran about 5 miles then died as I was driving down the highway.  I figured it was the alternator so I took it to the shop.  They said it was the alternator and replaced it.  However, the battery is still draining.  The shop thinks it is an electrical problem but doesn't have the system to check that so they recommend I take it to a dealership.  Also, I don't know if this is related or not, but when it rains, the truck backfires almost non-stop.
What do you recommend?
ANSWER: What motor is in it.

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QUESTION: a 4 cylinder
ANSWER: Check you ignition wires for the backfiring, 99% chance thats all that is. You dont need any special equipment to check for an electrical draw. A simple voltmeter will do just fine. I would stay out of that shop if they do not know how to use one.  A draw on the battery means that something is not shutting off as its supposed to and using the battery power. I usually connect a volt meter on the negative side of the battery and start pulling fuses until the draw stops. Then I look up the circuit and one by one eliminate the components attached to it. Its not easy to do.

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QUESTION: On the alternator, the mechanic said that most alternators have a voltage regulator built in but for some reason my truck's computer or something is making the alternator "supercharge" the battery.  He recommended that I not sit and idle for long periods of time. He said that would dry/burn up my battery in no time.  I'm not very confident in the mechanic's opinion after reading your answer.  Have you heard of this problem?

Answer
First and foremost you do need to stay away from that person. If you battery is being overcharged then you regulator is no good and you need a new alternator, PERIOD!! This can easily be tested with an amp meter and voltage meter. If your battery voltage is rising above 14.5 volts you have a problem. You car should be able to sit and idle until it runs out of gas or forever if you keep it full. I'm not sure where this is going as you stated above that your battery is going dead!! Is it overcharging or going dead?