Thanks Bob and all -
From what I've read so far in this group, there seems to be a fair amount of doers here, hopefully I can fit in.
I like the idea of using the Arduino for data collection. It's well suited for that. Would you use a network connection between the Arduino and the Pi? I have a Ethernet shield for the Arduino. I guess the trick would be to "send" preformatted telemetry packets to the RPi on either the AGWPE or the KISS network port? Maybe there's a way to use a serial or USB connection between the two devices?
The truck's current setup uses a Friendcom 5 W data radio and an Argent Data OT3M, running 24/7. The OT3M is kinda flakey, and appears to lock up regularly with power cycling or with the USB connection (to the truck PC) turning on and off. The older setup in the truck was the Friendcom and a KPC3+, which was much more reliable. I'd modded the KPC3+ to have all 5 ADC ports brought out, so I could get my fill of truck telemetry (Vbat, Tbat, Iload, Tcabin, Vload). The truck also sports a 100 W solar panel and a second, deep cycle battery, and it's been exceedingly stable for years. Batteries last as long as they normally do in the PHX heat.
I'll admit that the power consumption for this Baofeng/RPi approach is way higher than I get with the Friendcom and an OT3M (or KPC3+).. The OT3M pulls maybe 40 mA at 12 Vdc, and the Friendcom is around 75 mA (?) squelched. That's more like a watt, which is nearly 25% of the RPi/Baofeng setup. But, I can't seem to find Friendcom data radios any more, and 5 W is really on the low side of what is needed for APRS in the hinterlands, but the standby power consumption of a 50 W mobile radio prevents it from being left on all the time. (Makes me pine for the "good old days" of a single-channel xtal radio, sipping 20 mA in standby!)
Back to the RPi and Dire Wolf. My continuing confusion was due in no small part to not RTFM %^). Nonetheless, last week I finally did pay attention to John's write-up on the telemetry toolkit, and now I think I get it more. I am neither Python nor Perl savvy (I'd like to be), but now I think I understand that he uses Perl scripts to call Python routines to grab telemetry from the RPi I/O, format the message in Perl, and send to the Dire Wolf modem. So, I ordered one of the ADS1015 4 channel ADCs, like he uses in the example, and maybe I'll be able to get that up and running quickly when it arrives.