I don't think the fuel filler cap can cause a check condition on a diesel. That's a gasoline thing (evaporative emission control). However, the Check light often can be triggered by a water-in-fuel or low coolant condition and these may or may not have their own separate indicator in your coach. It's up to the coach builder to supply the secondary dash indicators and that varies by brand, year & model.
The codes (and much other info) is carried on the engine data bus, which is probably the J1939 standard bus on your 2011. You can get a module that plugs into the bus connector and read the code with a variety of devices, and you can also display a ton of other useful data, e.g. turbo boost, transmission gear and parameters, water temperature, oil pressure status, historical mpg, etc.. One way to get codes displayed is to install one of several engine monitoring systems, e.g. Silverleaf VMS or Coretronics TripTek.
http://www.rvtechlibrary.com/electronics/engine_monitors.php
Cummins has a code reader called Cummins Quick Check - see https://cumminsengines.com/quickcheck It's a pricey thing, though. Here's an aftermarket reader:
https://www.amazon.com/OTC-3418-Heavy-Duty-Code-Reader/dp/B001A4FKA0
