i would keep the 4.6 in mind, but the 5.4 is way better for towing, i would take a 5.4 over my 4.6 any day. the most ive had in my bed was 5200 and it caused the bed to drop a inch and a half or so. and i had to ease on my gas pedal pulling that weight. also, there are aftermarket programmers that can increase your torque which can help when it comes to towing things. it also can help your gas mileage, if your interested. look up sct or gryphon

 

Not enough information.

What type of travel trailer are you looking at? Check the owners manual in that truck for a true rating. With that truck you are looking at around a 5000 pound max. You have to keep in mind that whatever trailer you buy has a weight rating that is completely empty. You will generally add 500 to 1000 pounds of gear with food and whatever else. That plus a full tank of fuel in the truck and the weight of you and passengers all has to be added together.

Doesn't really leave you much room as far as trailer size goes. These things get heavy quick. If you have not purchased either yet I would recommend looking at the trailers first so you have an idea of what range you want to be in then give those numbers to your son so he has a better idea on what truck you will need and he can find you one that will better fit.
Sometimes it is good to put the cart before the horse.

 

I would stay around 5k if I were you, its a good looking truck and sounds like you got it for a reasonable price, welcome to the forum.