Wishing and hoping that RT adds something to give better SOC data has been going on for a long time and other than the possible reintroduction of Coach Connect it seems very unlikely that RT will provide a shunt based SOC measurement.
So, being stuck with voltage measurements as the only real alternative, I would just make the best of the situation and try to get the most accurate voltage measurement and maybe use a recording voltmeter to save a history. It won't take much data to get enough info to come up with some "good enough" indications of SOC vs Voltage for your specific system under your typical usage scenarios. Since the voltage does depend on both SOC and loads and the chargers, you are never going to get anything as accurate as a shunt based measurement but it will likely be good enough to provide a reasonable SOC estimate for purposes of managing the Ecotrek batteries (when you need to bring fresh ones online for instance) and when to start battery charging. I have two recording multimeters that come in handy for this type of data collection, a MooshiMeter and a Redfish, both of which can record voltage data and transfer it to your phone, tablet, or computer for analysis.
In terms of the Voltstart trigger voltage, the trigger seems to be set nowhere near 10-20% SOC but this doesn't really effect the performance of the system. The risk of setting the trigger voltage too low is that voltage drops from load spikes could trigger the Ecotreks to go offline or the inverter to shut down. There is less risk of this at higher trigger voltages. You get the same energy out of the system at a higher trigger voltage as long as you don't go too high and run the GU when the batteries are at full charge. In the end, you can get the total energy available in the batteries and the energy provided by the GU during the Voltstart cycles at trigger voltages well above the 10-20% SOC which is where most expect the trigger to be set. Again, if you set the trigger voltage too high then the GU will fully charge the batteries during the Voltstart cycle and you would waste the remainder of the GU runtime energy. Of course, the RT and salesmen talk at times about the Voltstart having the capability to keep you batteries fully charged during the Voltstart operation but that is a bunch of BS and would be counterproductive if it was designed to do that. You would need to trigger the Voltstart at very high SOC and likely lose a lot of available GU energy charging full batteries.