Diesel is best for RVs.

Diesel is definitely NOT always better. The cost of ownership is very high on a diesel. Yes they get better mileage and generally they do last longer but they also cost more when they break down and need major maintenance more frequently than a gas engine. The answer is going to depend on if you absolutely need the towing capacity of a diesel or not but if the V10 will suite your needs I would recommend going that route. 

Do you know the model of the Duramax?

There are a lot of them on the road, the 6.6l Duramax is one of the most reliable diesel motors available today. Provided proper maintenance, it can easily hit the 350,000 mile mark, with some users claiming 400-500,000 miles is the norm.

There have been a couple of models that experience some issues that can be solved by modification or knowing the root cause(s).  You also get the better mileage.

the Ford V10 has a stellar reputation for reliability. Assuming the engine has been well cared for throughout its life, you can expect it to last at least 200,000 miles before needing to perform any major repairs.  The one thing to watch out with this motor is the spark plugs.  They are short and go into aluminum and you have to be very careful changing them.  They can have issues with the spark plug blowing out as well.  Then you can not put a plug in. 

In this case I would go with the Duramax at 68k if there was any issues they would have been sorted. 

Hope this helps a bit