You should download the manuals for all of your equipment. They will give you maintenance intervals and suggested service. Most people change their oil and filters at least once a year, regardless of what the manual says. I've always found the generator easy enough to service myself on all of our rigs. Annual service on your engine should cost less than $1,000, and depending on what you have done can be much less.
If you haven't already, you'll want to register with Cummins Quickerve. All of your manuals and a ton of other useful information is there and it's free. https://quickserve.cummins.com/info/index.html
Many RV dealers will provide service for most of your equipment, but they can be slow and sometimes they lack the quality and certification of a Cummins service center. Cummins is usually faster to get to your rig also, although that depends on where you are. They can service both your engine and your generator.
Another good option is factory service. You don't say what rig you're driving, but most of the brands have service centers that can handle about anything on your rig all at the same time and many allow you to camp there and have other conveniences. Tell us what you're driving and others with the same brand and model will chime in. It's also handy to add to your signature line. Good luck!