The final touch on my service body will be a series/parallel switch between the under-hood batteries and the service-body battery. That's a $975 relay plus a $125 solenoid controller:
http://www.gonefcon.com/trucktcom/parallel_sw.htm
http://texasindustrialelectric.com/T_114_12_series_parallel_switch.asp
I have a NATO receptacle and slave cable coming. The receptacle goes on the service body. I'll be able to deliver around 1,500CCA @24V to jump-start my "future parts SEE" even if its batteries are dead or missing, by just driving up to it, hooking up the cable, and flipping a switch. 24V trucks & tractors are common around here (I already mentioned my neighbor coulda used a 24V jump-start earlier this winter when his tractor's block heater failed), although the newer they are the more likely they're hybrid 12V rigs with 24V starters. What I couldn't find were any junkers I could strip the series/parallel switch from. I'll need a small $15 relay to keep the solenoid from engaging unless there's an "engine run" signal.
Those timers I bought will be returned for refund next week. Oh, they work fine, just not "as advertised." Instead of a range of 1 sec to 100 hrs, they're really 1/10th sec to 10 hrs. The mfr. solution is if you need more than a 10-hr interval, you can pay to ship them back to the supplier for a refund.
At least I got all the wiring mocked up, enough to realize how two of these timers won't stay in synch regardless of interval. So I've ordered two "13801" timers:
http://waterheatertimer.org/pdf/Hager-analog-module-sell-sheet.pdf
(wish they had a colder operating-temp rating, we'll have to see how it goes)
One for the Dodge, one for the SEE. The one on the Dodge will run off 12V, but I'll wire the one on the SEE for 24V so it doesn't just drain the first battery. From there, on the SEE, the current passes through one of those thermostats I have here (they work great), controlling a solenoid:
https://www.stego.de/en/products/regulating/regulators/nc-heating-or-no-cooling-standard/
https://www.stego.de/en/products/regulating/regulators/relay-24vdc-and-48vdc/
The output from that solenoid will pass through a variable on-delay relay, so I can set the hydronic heater to fire up long after the AC heat elements without worrying about keeping two timers synched:
https://www.ntepartsdirect.com/ENG/PRODUCT/R60-11AD10-12
The current will also go to a solid-state relay to switch on the AC current:
https://www.ntepartsdirect.com/ENG/PRODUCT/RS3-1D10-51
I'll be sure to take some pics of the installation as it progresses. Aside from the wrong timer and relay, the mockup's already done, with the added complexity of the step-up converter & more-complicated AC wiring for the Dodge. I hope these timers work out, it'd put an analog clock in the service body, plus one in the cab of the (clockless) SEE, between the seatbacks. The thermostat's set-and-forget, but it should be easy to access & re-program the timer, or hit its override switch if it won't be used for a few days or whatnot.
The upside to the defective timers, is a more simplified setup that's intuitive to operate.