Actually, determining the towing capacity of your F-150 is not a guessing game, but you have to know where to get the proper information. You should be able to get that by giving your VIN (number on the dash) to your Ford dealer and having him look it up. There are many different configurations of a F-150 with a 5.0 liter engine.
Or you can get a pretty good idea by Googling Trailer Life Magazine's Towing Guide. They have guides online going back to 1999. If you look there, you will see towing capacity for the 2020 F-150 with the 5.0 L engine, for example, that ranges from 5000 to 9600 pounds. Depends on things like the transmission, 2 or 4 wheel drive, the rear axle ratio (which can range from something like 3.13/1 to 3.55/1) and so on.
Then, if you read the fine print, they are making certain assumptions. They assume your truck is fueled up and is carrying a driver and one passenger, but no cargo in the truck at all. They also assume the tongue weight is 10% of the weight of the trailer. They say for the maximum capacity you have to be using a Weight Distribution Hitch. But the weight of that hitch is not included in the calculations, so you have to adjust the tongue weight and weight on the tow vehicle accordingly.
As pointed out by others, the posted weight of the trailer pretty much assumes it is empty. It may include the weight of full propane cylinders, but it doesn't include the weight of anything else you load into the trailer, including fresh water, clothing, food, etc. And options and other improvements that may have been added to the basic trailer. So you have to estimate all that when figuring out what your trailer will actually probably weigh.
But you won't really know what you have until you buy the trailer and tow vehicle, load it all up for a camping trip, and take it to a scale and find out what the truck really weighs and the trailer really weighs. By axle. If you know all that, you can have the dealer look up to see if you are in the weight limits of each axle (and wheel and tire) on the truck and the trailer. (You can find out the limits of the trailer axles, wheels, and tires if you are diligent enough as well--sometimes the trailer manufacturers don't put heavy enough equipment on them for the realistic weights they will carry.)
So who goes to all that trouble? Probably not many RV owners. As a practical matter, once a person has estimated all the weights, it makes sense to have a tow vehicle that has a extra capacity beyond what you will need for a particular trailer. I'd be more than happy with a (loaded) rig that would tow 20% more than I expect my trailer weight to be. That way, I need not worry about throwing a chain saw in the truck bed or taking a couple of grandkids along. Or bringing a few rocks back for my wife's rock garden, which I always have to figure in.
It all comes down to educating yourself about what you have and what you should have and using some common sense.
That's my two bits worth. At least for today.
1999 22L Classic
2008 Toyota 4Runner V6