Hey there volstag, check those 150 Mhz freqs for AAA dispatch, Sonoma County will definitely be on one of them. That will only be the dispatcher for AAA calls though, and the tow trucks will also use company freqs, most likely on a talkgroup on a local LTR system. Do you have an LTR capable scanner? The likely places you'll find these systems on are:
450 Mhz - 455 Mhz
460 Mhz - 465 Mhz
480 Mhz - 485 Mhz (Remember Marin has their MERA system in this range so you'll hear a lot of their digital traffic from the Sonoma Mtn site)
Last time I was up in Petaluma I found what I think was Bay Bridge Garage Towing on this system on Sonoma Mountain and submitted it:
http://www.radioreference.com/modules.php?name=RR&sid=4624
There could be more freqs associated with this system, and there are also LTR systems on Mt St Helena, Mt Veeder, and elsewhere, that would be likely candidates for your tow truck companies. If you can scan LTR systems, go to the Sonoma County database page and plug in every trunking system that says "LTR Standard" (no scanner is made that can trunk-track LTR Passport, so if you want to listen to those you'll have to in conventional mode and won't be able to tell who's talking).
Next is the fun part: There will be all sorts of buisness users on these systems (cabbies, tow truck drivers, trucking, private security companies, general contractors/HVAC and plumbers, etc. in addition to tow trucks) so you'll have to listen and figure out which talkgroups sound like they have tow truck traffic on them (some use T codes like a T6 for tow, or just plain english). This in itself isn't that hard, but since you want to know the channels for individual companies, keep listening to the talkgroups you've identified as tow trucks, and see if they mention the company by name. Usually they don't, so then you have to do it the hard way - Start listening to the addresses their calls are at and plugging them into google maps or mapquest. From this you'll be able to identify at least the general area or city that company is localized in. Then search online (google or yahoo yellow pages) or in the local phone book and see if you can find any matches geographically.
If you get more than one, or you're really stuck, you could also try listening to local law enforcement. I'm sure you do that anyway, but when they call for a vehicle to be towed (11-85) the dispatcher will usually call back with what tow company and their ETA. When you have the location of the call and the company determined, start scanning those LTR systems like mad - if you find some angry lady at the garage barking orders at a driver that it's a PD call, and the address matches what you heard on PD's freq, then you've found your towing company!
That sounds like a lot of work, and it can be, so it really depends on how much you want to figure out who's who. Out of bordom with routine law enforcement and fire traffic I've scanned around looking for local taxi and towing companies, and have gotten extremely good results using the above method, so I just wanted to share it.
Good luck!