The hoses to the coolant filter are standard 3/8 inch black hose unless they have been upgraded by the owner. The one that fails most often runs from a fitting below the alternator (you have to remove the alternator for access) and down the side of the engine above the manifolds inside a heat protective shield and around the rear of the engine (the rear of the engine is towards the front of the coach) to the filter that is mounted to the frame on the other side. The other hose runs around the rear of the engine to a fitting on the rear of the intake manifold. If you change a hose, change them both, because I have seen coaches that have had them both fail. I know of one coach that has had two failures that totaled nearly $2,000 in towing bills, and believe me when I say that you do not want to have your coach towed if you can possible avoid it.
Some shops will change the coolant filter to an empty shell (dummy filter) when they change the coach over to ELC (extended life coolant). This is a bad idea because the hoses to the filter are such a high maintenance problem. As for an alternative routing option, there is not a good one because of where the hoses connect to the engine.
On the topic of other maintenance issues, this thread is about C12 coolant issues and the only other thing that I can think of is the coolant surge tank. The surge tank on these engines and many other Beavers in this age range is a Ford tank that cracks often. The Ford part number is F6HZ*8A080*B and it measures app. 3 1/2" deep, 6" wide, and 27" long. At the Indio rally there were 3 coaches with cracked tanks. I will address this in another thread.
As for other issues. they will need to be addressed individually instead of here since the C12 engine was used in Marquis and Thunders over an 8 year period, and the problems were not the same with all of these coaches. Just like the brake problem that Ed mentioned covers a little over 4 years of Marquis production, but not the Thunders and not all Marquis with a C12 engine.
Gerald