You don't need anything sophisticated unless you have a healthy budget or uber R&D skills. BUT you do need to measure your battery capacities.
How will you know much margin you have to start the diesel with any SoC of any Battery design?
What is the consequence of failure?
How will you connect the two batteries with a known CCA rating but unknown battery health and SoC? What is the max surge current expected if one battery is charged 100%SoC and other one dead at 0% SoC?!?
Since batteries have a low ESR like that of the starter DCR you can expect similar surge currents on connecting them with different SoC voltages.
CA and CCA ratings are done at 7.5V while Ah ratings are done at a 20 h rate. ESR of battery rises sharply as battery drops below 10%SoC thus voltage during "cold start" drops more with low ; SoC, battery health, age, temperature , dry cells and low s.g.
An inexpensive s.g. tester for cell acid is advised too for testing mismatch % in each cell as a battery health and SoC indicator. New is <0.1% matched , while Dead , shorted , or sulphated can rise to >10% mismatch.
There are many solutions to each problem using dual diodes, starter solenoids, smaller transfer relays to bypass the active solenoid so they can be shutoff and only used to connect and disconnect the batteries.
You need a few tools, specs and skill to do this right; plus an emergency solar panel or generator source like a motorcycle and current limiter.
A cheap DVM may be powered by the car battery with a 9V LDO to monitor both batteries.
You need to estimate Ah SoC using a Voc chart after a short term load or long wait and you can test each battery for Voc vs SoC, normally 12.5 to 11.5 for 100% to 1% SoC. Then measure battery Ah and voltage during battery drain over a 4 to 8h period and expect 10 more for a 40 to 80hr period.
THEN plot this Ah consumed vs V and Voc for each battery.
motor start at various levels from 10% for starter reliability then charge it by adding 10% of actual Ah capacity (not just rated)
You can simply use a DMM and keep both batteries in tandem and use rear access for camping DC outlets, with jumper bridge cables or smaller gauge wire based on a safety breaker rating that you add-on. Then you test and decide how much margin you need between charging.
50% SoC is near 12.0V as a rule of thumb but you battery health may affect this due to a weak cell.
20% SoC may be a safe lower limit depending on your rig.
A small Solar Panel may be a safety net.
A typical van battery "might " be around 50 Ah or more when new.
Keep in mind Relays are rated for much lower loads in DC than AC but Automotive Relays are cheap and will conduct higher current if not switching it.
Starter solenoids may be used to connect batteries momentarily then bypassed with a lower Amp rating Relay or suitable switch.
Automation is the easy part, but testing and design specs are the hard and necessary part for reliability.