New Sprinter Era owner

might be for air ride shock,s or air bag suspenion. call a dealer and ask.

Could it be the sensor for the outside thermometer?

Book, is yours a 2013 chassis?

Yes, 2013 Era

Yes, 2013 Era 6/12 chassis ?

Fuel gauge question ... Warning light comes on
After 340 miles..... Fill up only allows 20.7
Gallons? Is reserve really 5-6 gallons??

Many folks have commented on the fuel light coming on at about 20 gal of fuel used. The tank is 26 gal so it looks like there's plenty of "safety zone" fuel left in the tank when the light comes on. I believe your unit is normal with regard to the fuel light!

A diesel has a lot of problems if it runs dry, far more than a gasser (and even with gasoline engines, it doesn't help the fuel pump any.) I'm pretty sure that is why the Sprinter's fuel light comes on at 1/4 or so.

The 26 gallon tank also has an air pocket for the gasoline to expand in. Realistically, it holds about 24-something gallons.

Book, your ERA is a 2012 chassis, which is not that unusual for an RV to be one year chassis and another model year RV. Sometimes there's 2 years difference.
It sat on a dealer's lot for awhile after being manufactured.

And the low fuel light does come on rather early at about 6 gallons left. Normal Sprinter operation. We generally don't let ours go below 1/4

The Sprinter holds the 26 gallons. I've put as much as 25 gallons in driving well after the light came on. The light comes on when the onboard computer readout deems you have about a 90 plus or minus mile range left. That's safely close to 5 gallons I would guess. On the needle gauge that is just about right after the needle goes to slightly or barely still touching the less than a quarter of a tank mark.