I can't address generators because I have not done them, but that's a good option.
Here's what I did with a battery equalizer and have had no problem. There are notes and wiring diagrams to help. The article goes into more detail than most for adding 12/24V power, so it should be sufficient to help. Ignore the VIC-4 intercom part, it sort of ended up in the same page because I am a terrible webmaster and a mind full of convoluted thoughts.
http://www.bunkerofdoom.com/mil/m35/vic4/index.html
Here also are my 2 cents from experience:
If using a battery equalizer, note that it will probably draw a small amount of power all the time and eventually will run down the batteries. A solution is to use a double pole contactor with a 24V coil and have it switch ON automatically when the truck power is turned ON. The article details all this. (a 2-pole manual switch or breaker would also work)
At minimum I'd reccommend the military type 24V-12V conveter mentioned, and if its output voltage is actually between 12.6 and 13.6 volts, you can put a spare 12V battery there to handle your big surges. The battery will last for several years if the voltage is kept in that range, the only discrepancy on voltage is how much charge is in the battery, which will be immaterial for most purposes because whatever was taken out suddenly will eventually be put back in by the converter.
35 amps is enough to run a 200 watt RMS car stereo at full volume continuously. As we know, music volume is not continuous but consists of relatively low power levels with occasional big surges, so in fact you could operate a 1000 watt stereo off that if you wanted because the average power is lower and your added battery would take up the slack on surges (assuming the 24-12V converter has built in current limiting, and most do). Don't waste money on 1-farad capacitors.. use a battery.
The best and most expensive solution is a battery equalizer.
Unless you are running some high power sensitive equipment (not car stereos..), it won't matter what you use, as long as you either use a properly sized 24-12 converter or a battery equalizer.
For the main (between the battery and everything else) switch on your 12V system, a magnetic DC circuit breaker is the best solution. I have been using 100A ones for main switch, and then each piece of gear has one rated for that piece. Beats paying for fuses. You can also use house light switches plus fuses but they are soooooo tacky. I had a bus with those nasty things and first thing I did was rip them and the fuses out and put in mag's.
Finally, you will probably want to put a commercially available noise filter on your 12V line because the truck's electrical system is noisy (fuel guage, alternator, flasher, instrument panel and senders, heater motor) and some of the noise may be reflected in the 12V system.
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