We have a '93 Toyota AWD Previa with 193K miles on it (and a rebuilt engine at ~140K after pulling a tent trailer to Banff, Canada and back). It has been a wonderful vehicle, but on our last long trip (Santa Cruz, CA to McCall, ID) the alternator quit the day before we were going to head home. It took the mechanic 4 days to find and get a replacement shipped to McCall, and we ended up driving back to CA overnight to get our daughter to school on time. My wife is now unwilling to take it anywhere except around town, and a friend of mine (who knows more about cars than I do… which does not take much 
told us that the transmission on these typically last 150K miles. So my question is: is there anything we can to pro-actively to make this reliable for long trips (e.g. replace the transmission even though a local mechanic checked it out and said it was fine), or should I give up on it?
A failed alternator is hardly a good reason to give up on a vehicle. I’ve been to McCall on a ski trip to nearby Brundage Mountain and it’s a small town. I can see where it might take awhile to get parts. That said, given the age and mileage of the van you can expect a few problems from time to time. It’s almost 20 years old. Regarding your transmission, make sure you change the transmission fluid and filter at the recommended intervals. Aside from that keep up your maintenance on the rest of the vehicle as well. Keep in mind that some parts for older, low volume vehicles can get harder to get as time goes on although it’s hit and miss.
The way I always look at it is, the car can care less if you put 1000 miles on it in a day or a month. If something is going to go wrong, it’s going to go wrong. So if you trust it around town, trust it in the road.
My aunt actually felt the way your wife felt about her van as well. With 180,000 on the clock she was renting cars to drive long distances. I told her the something I posted above, and she has since been on 3 long trips with her van with no issues.
While your car SHOULD last quite a bit longer, once problems start, the trust is gone. The other issue is the difficulty finding parts for it, as you know. Finally, a newer vehicle will be safer in an accident than your '93. You’ve gotten a lot of good use out of it, I’d look at replacing it. But that’s me.
I think what your wife is saying, is “I want a new car”. Your Previa is as reliable as you can expect from a '93 which is now a 20 year old car. A new alternator is really no big deal. You should be on a 2nd or even 3rd one by now with those miles and years.
You have lots of 20 year old parts that could fail. I’d consider the condition of the radiator and all the coolant and heater hoses. Belts, hoses, radiators, and tires are most common things that fail on long trips. Sometimes even new cars break down on trips, so you can never be totally sure of any vehicle. At least a newer car is more likely to have parts more readily available.
There were never many Previa’s on the road to begin with, and some parts for a 20 year old car can take awhile to source. I’d say you are likely good to go on many more trips in the Previa. But, if the wife has lost confidence in the car, then next time you are on the side of the road you can expect to hear “told you so” once or multiple times.
Any 20 year old car with almost 200,000 miles on it will have something break occasionally. An alternator is nothing major. This is not an indication that the vehicle is anywhere near worn out.
I think Uncle T said it all perfectly.
Transmissions and engines in these vehicles are bulletproof. I owned one for 220,000 miles, sold it to a neighbor four and half years ago and I still see it running around town with over 300,000 miles (no engine or transmission failures). The downside to these vehicles is that a simple item like a starter or fuel pump relay failing could lead to being stuck somewhere for days while waiting for parts to come in. The advantage to a newer vehicle is that a breakdown is not a catastrophic problem. I miss my Previa but my Sienna is safer, quieter, and easier to work on and is MUCH more luxurious.
You can’t feasibly make a car that old with that many miles reliable. (You’d have to replace too many parts.) Either accept that it’s more likely to break down than a newer car or buy a newer car.