Jerry - There have been numerous discussions here in the Forum about the Jake Brake versus RPM issue. Within a year of my purchase of my Marquis, I had the Kenworth shop in Pasco, WA reprogram my transmission to NOT shift down to 4th gear when the Jake Brake kicks on. I had Kenworth program the transmission to NOT shift out of 6th or 5th gears unless I do it manually, or as it normally does under normal driving conditions. Now I don't worry about it causing a spike in the engine RPM.
My C-12, 505 HP engine rarely goes above 200 F. Almost always the C-12 temperature runs between 185 F and 196 F. On the other hand, my transmission rarely goes above 140 F. The transmission runs about 40 degrees over outside ambient temperatures. Only on a hot Summer day have I seen my transmission temperature near 140 F. Yes, it has an added cooler for the transmission according to the previous owner's remarks.