You will get a myriad of opinions
1) I agree with the above. Tow it with your present F150 and see what it feels like. Do you have a trailer tow package on the truck?
2) You might want to personal message Andy at Can AM. He's on this forum. He stated that with the right hitch set up etc an Expedition would work fine for my 28' Serenity but you have a 31'.
3) How much are you going to be towing? The more you are towing the more likely you will want a beefier TV for payload purposes since you will want to carry more things.
4) Where are you going to be towing most of the time? If you are towing a lot and mostly in the mountains then you might consider a 3/4 ton diesel with the engine break for safety. Not saying you can't tow with gas 3/4 ton. Many do it just fine. New are very expensive. Good used are hard to find. Affordability?
I tow with an 2017 F150XLT 3.5 Ecoboost Crew cab with max tow haul package. I also have a propride hitch which greatly adds to the stability while towing. Some of the things that can be added to the F150 are better shocks (bistein) and better towing tires. I do know that the new F150's come with a better tow rating. BUT the issue then becomes payload. The more options you add to the F150 the less payload. Just remember that if you order one
Another person on this forum bought the F250 gas. Looked like a very nice truck. The thing about gas is that it is cheaper to fill, cheaper to repair, cheaper to maintain, and cheaper to buy. I've heard the break even point for the diesel is about 200,000 miles (I may be wrong). The bad thing: Not as good a ride; although some contend they have improved.
1) Where are you going to tow?
2) How often are you going to tow?
3) How much can you afford?
4) What else are you going to use the TV? Daily driver?
I don't think there is one right answer, but I do believe the more you tow, the more extreme conditions you tow in etc could possibly tip you to a 3/4 ton. Whether gas or diesel the question now becomes, "how much can you afford?"
My only concern with the 2018 Expedition is payload. But if it's just you and your wife and you are taking mostly short trips etc. I think it would work. PM Andy at Can AM.
Good Luck. My 2 cents.