glow plugs or likely the grid heaters are a huge current draw but at least with glow plugs for a very short period of time, seconds or so. I've measured the peak draw on my duramax compared to starting current, and glow plugs draw more than the starter.
So, a marginal battery can get hit by the glow plugs or grid heater.
Now that said, my 2004.5 duramax batteries lasted till mid 2014, and the replacements are still ok. Replaced them with OEM delco.
My 2015 batteries are doing fine, I expect them to go much longer.
Best thing to do is to do a load test on each battery by someone who has a real carbon pile load tester and knows how to use it.
BTW, My 97 sedan is on it's second battery, My 97 roadster is on it's second battery as well. Yes, over 10 years on each battery.
Now we don't get the brutal heat some areas do and heat degrades batteries more than cold. And we don't get brutal cold like the OP likely does in Idaho.
But still, 3 years on the batteries seems to be on the low side for today's batteries.
2011 Keystone Outback 295RE
2004 14' bikehauler with full living quarters
2015.5 Denali 4x4 CC/SB Duramax/Allison
2004.5 Silverado 4x4 CC/SB Duramax/Allison passed on to our Son!