As long as the tow ratings are sufficient for the TOAD you plan on, either should be fine. If you are planning to pull a big TOAD with big MH, it may exceed the tow ratings of the gas engine, then the answer is obvious.
If you will spend a lot of time in the mountains, a case can be made for diesel as the turbo keeps the power up at altitude and an engine brake is nice for coming downhill. But if it's just an occasional trip at higher altitude the V10 will do fine.
Given your mechanical history (and assuming you would do much of your own work) if price was similar, I would lean diesel as much of the costs could be addressed by you doing your own work but if there is a big cost difference, I would go with the lower cost option.
As far as life expectancy, there is no reason a well maintained engine (gas or diesel) won't last 250k miles. By that time, the body, suspension and rest of the unit is probably junk and not worth fixing. Assuming you stick to something like 12-15k miles per year, that's a long time. Most MH's die from lack of use not overuse.
