They are all reliable now but just like anything else made by man there is always the chance of a breakdown. If it's out of warranty than yes it could be costly but so could a gas motor if the right part broke.
If you go around worrying about this stuff it is going to eat at you all the time and take the enjoyment out of your trip.
If you want more truck than what is in your siggy then you could just move up in truck size and stay with gas. If you want a diesel just because you want it then make sure you can afford the issues that may come with it.
When I bought my '05 Ram Cummins all I heard was the trans will go and the frontend wouldn't hold up. I'm still driving it on the original trans and frontend parts. Well over half the miles on it are towing miles and most of those are with a programmer turned all the way to max setting. Just take care of what ever you get, don't try to make it do more than it can do and it will last you a very long time no matter what you get.