While I realize the 7.3 is more powerful, we've had two of the V10 F250's and with a 12k 5th wheel, it was downshifting at every overpass, so even though 800miles of Michigan isn't a lot of tough mountains, if you get up around 16k, I would have preferred the diesel. Down around 13k, the 7.3 is more reasonable.
Yes, it will pull the 16k (assuming the ratings allow it) but it will be turning a lot of revs any time you get in hills.
On the positive side, gas is better in terms of payload limits of a 5th wheel since it's lighter than the diesel.
In terms of safety, once you get on the road, the length is largely irrelevant. It's when you try to put it in a tight parking spot it can work against you.
For us, it's the smallest RV that we can be comfortable in. That means a little 20ft trailer is unacceptable but we certainly don't need a 40ft monster.