The CAT ELC coolant is good for 300,000 miles or 600,000 miles after the addition of a boost at 300,000 miles. No other maintenance needed.
First, Native, I mean no offense here. I have no issues with you, so don't take this as coming in guns a' blazin'. Simply pointing out a few things I am privvy to.
Something that should be taken into consideration with OAT ELC (or any ELC, for that matter) is that it is designed for an entirely different set of parameters pertaining to operation and run cycles than what a skoolie conversion will be subject to.
This stuff is designed for heavy-duty diesel engines that generally idle a lot and have fewer start-stop and cool-down cycles than what a skoolie conversion subjects its coolant to. This is vital to activating the Organic Acid Technology that 'extends' the life of the coolant, which greatly changes the scenario in a vehicle not operated accordingly. It would appear that OAT requires a narrower window of temperature in order to work as advertised.
These engines also accumulate far more mileage far more quickly. When I was driving over-the-road, I averaged 550-625 miles per DAY, running solo. I've been known to make 700 miles on a perfect day ( didn't get caught though... :whistling: ) A truck driven by a team (two drivers running night and day) could easily run 1000-1300 miles per day.
Putting this into perspective, 300,000 miles goes by very quickly under those circumstances, far faster than it would with a skoolie or a local truck.
A team truck would require changing the coolant at roughly 225-250 days.
A solo truck would require changing the coolant at roughly 480-550 days.
Even for full-timers that travel 1000 miles per week, that translates to 300 weeks, or 2100 days. 2100 days is nearly 6 years. The rest of us might put 150 miles a month on our rigs. This translates to 2000 months, or 166 years...

Also consider that the coolant largely not remaining at a suitable temperature to keep the Organic Acid Technology active, will cut the service life of the coolant quite a bit. If I had to venture a guess as to the difference, I would say 2-3 years.
Remember when GM had all their issues with Dex-Cool? That was, essentially, Extended Life Coolant, though I am not sure if it contained Organic Acid Technology.
Customers were told 5 years or 100,000 miles. So people believed them. And after 2-4 years, intake gaskets were blowing, plastic intakes were corroding, and engines were being ruined because this stuff was turning to acid after 2-3 years. I strongly suspect the difference in operation was at least partially to blame, if not entirely. I know the formulations have changed since then, but I still don't think I would trust the claimed service interval for a vehicle that isn't run or driven much.
Hence, I say that the 200,000-300,000 claims for OAT ELC for a skoolie with an engine requiring this coolant be taken with a grain of salt. Just my $0.02, but I do have experience in commercial trucking and have spoken with several diesel mechanics about a great many things, this being one of them.