Gary,
Overheating is always a topic on some of the other forums, ( Diesel RV, Discovery, RV2) seems like cleaning radiator annually is not uncommon. So if you are getting 18 months, that is probably pretty good.There is also a lot of disscussion about the slober tube and where it vents. I would suggest going to the Diesel RV Club forum. There are a lot of great tips on overheating and other diesel engine topics and RVs in general.
Jim Pinkerton
06 Discovery 39L