alternators

Go and speak to your friendly auto electrician or if wanting to do it on the cheap visit the wreckers.
Basically you can fit a higher output alternator that has the same mounts and fittings, just bolt it in and away you go, best to get it off a later model Navara if they are similar to avoid problems.
BUT there are a few things to watch out for,
1/ make sure that the chosen alternator has a higher output at the same revs as your existing one, some high output alternators put out less than lower rated ones at low speeds
2/make sure pulley section size is the same or it will not fit.
3/ make sure the dia of the pulley is the same or the alt will not run at the correct speed.
4/ If someone tells you that they can rewind your alternator for a higher output it might be best to move on as most alternators that i've come across when rewound for higher outputs need more revs and actually put out less at low revs than the original
I'd visit an auto electrician first as they will know what is a bolt in replacement and will avoid a lot of grief.

jessev,
Had the same model for about 12 years with the same problem. On country roads at night with driving lights and fridge running the battery would go flat and lights got dim. Had to run low beam for a while to charge it up. Dual batteries wont really help much. I put in an 80amp alternator. I think it was a standard borsch out of a holden or something. Worked excellent, never had any problems since. Only issue to be aware of was the auto choke and fuel cut off worked off the alternator which gave an idle speed of about 2000rpm. Took it back to the auto electrician and he fixed it up straight away. Best and cheapest way to get one is to take it out yourself and take to an auto electrician and get an exchange one. They will need to take all the measurements of its physical mountings which can only be done off the car.
New alternator may also get rid of any choke issues when car is cold.