Oil/Filter and the fuel filters once a year of every 15K miles (I think). Check with your coach documents or call Cummins.
Transmission, again check the coach documents or call Allison. I think the Transynd fluid is due for change at 150K miles or 48 months.
The chassis will need to be greased.
If you have green coolant, the SCA (Supplemental Coolant Additive) will need to be checked. I think the green coolant requires changing after 36 months. If you have Final Charge (or any extended life coolant) the coolant is good for about 300K miles or 36 months. With Final Charge one adds a quart of their extender and your good to go for another 3 years. This is a total of six years.
As to a cost, I do all this work myself. I have seen signs at my dealer offering these services at several hundred dollars for each service.
If you have a Suburban HWH the anode rod will need replacing every couple of years.
The generator has similar maintenance as the engine. Oil/filter, fuel filter(s), air filter and coolant. On average, once a year should be okay. Consider putting the same coolant in the generator as is in the engine. The maintenance schedule will be the same for both.
Check the roof seams. They may need some maintenance. I just cleaned the roof seams really well and put Eternabond on all the way around.
2005 Newmar KSDP 3910 + GMC ENVOY XUV 37K lbs Moving Down The Road
The Avatar Is Many Times Around The USA
Nobody Knows Your Coach Like Somebody Who Owns One Just Like Yours