IIRC, I have a Suburban in my 233s and I don't recall ever seen an adjustment on mine. I think that most HWH with DSI come with a preset thermostat. Maybe you can retrofit something from suburban.
FWIW, we had a camper in the past with a gas only water heater - no direct spark and no electric element. I needed to light that by going outside and light a pilot. That heater had a dial that I could move between warm and hot.
DW, 3 Kids and our Goldens
2012 Shamrock 233S
2008 Toyota Sequoia 5.7L 4WD