Spider, cruise controls and exhaust brakes on these units do not all work the same. As an example, my coach (2007 HR Endeavor, Cummins 400 ISL, Alison 3000MH transmission) the CC does work with the exhaust brake on. If you have the Aladdin monitoring system you can use it to check if the system is working. On my system Aladdin main monitoring panel I have a line for cruise control and it shows the set speed and CC status - ON or OFF. I thought my system was not working several times before I discovered this feature and began to monitor the CC function. There is frequently a lag between your input - SET, RES, speed adjust, etc - and it actually taking effect. I was going to complain about mine until I finally watched the Aladdin and saw it was working properly.
Another thing I have discovered on my coach and have not pinpointed how it works is the exhaust brake and CC coupling. In most cases, with the switch on, the exhaust brake will not engage when the CC is on but there are times going down a grade, with my foot off the accelerator, when the speed builds up to about 75 mph, it WILL engage: activate exhaust brake, select transmission to 2 and down shift according to profile. I don't know if this happens because the CC shuts off or if this is a feature of the exhaust brake and CC combined. So far I have been pretty busy driving the MH in those situations and have not looked at the Aladdin but I'll figure it out eventually. The bottom line is that all coach CC and exhaust brakes are not the same so I am not sure what you have and how it should behave. Hope this is helpful.
2016 Winnebago Cambria 30J