I have the same furnace
and a Suburban thermostat. Mine does have temperature markings on the case. It seems to work just fine. I did have to replace it and I believe the new one didn't have temperature markings so I just used the old cover.
There lots of room for questions for "seems to shut off at 70F". What shuts off, the flame, the fan? Where's the thermostat located? What's the outside temperature?
Explanation of questions
The fan doesn't shut off until sometime after the flame is shut off. This is to cool the fire box a bit with air moving over it. The thermostat should NOT be located in the air flow from or going to the furnace. It takes a while for the trailer to cool down once it's warmed up inside(insulation doing it's job). If the outside temperature is too warm it will be difficult to test the thermostat setting.
If you're getting cool nights set the thermostat near the minimum, just barely off the lowest setting. Check it in the morning, the temperature should be around 50F.