You're certainly not crazy.... or if you are... at least you're in good company around here.
You're right to be concerned about the cost of ownership on one of these beasts. It can add up quickly. Extended warranties might provide some peace of mind but they are expensive on older coaches so it's a matter of how much risk you're willing to assume.
I just had a basic oil change done on my Cummins at a local Frieghtliner shop and it cost $344 for just lube, oil and filter change. This has to be done once per year on most modern DP's.
In addition, there are more comprehensive chassis services which need to be done every three or four years... depending on how much you drive. These services can cost around $1100.
Many folks do much of their own service work, but I've chosen not to, even though I probably have the skills. I like having Freightliner give my chassis a good once over every year.
If something breaks outside of the normal maintenance, it can be expensive. Seems broken things on diesels can really hit the pocket book hard.
Don't overlook tires when making out a budget. Six tires for a DP can cost between $3K and $5K and they need to be replaced every seven years or so. That adds $500/$700 per year to the cost of ownership.
It can be frightening when you stare at these costs but it's best to go into this with open eyes.
Best of luck