There are 4 vitally important things that must be checked before you start installing your own 5th wheel hitch. They'll seem obvious to a seasoned camper, but without them, it's all too easy to completely mess up a hitch installation and have an epic disaster on the road.
First: Buy the Right Hitch for Your Vehicle
Your hitch is going to be towing around a lot of poundage. With the wrong hitch, you'll find that the weight you're towing doesn't get transferred to the ground in a balanced and stable manner. Buying a generic hitch that isn't made to work with your vehicle will 'usually' work fine, but it basically means you're pulling part of a heavy load based on the strength of some sheet metal -- worth the little extra effort to get a hitch that's purpose-made to work with your vehicle.
Second: Can Your Handle The Weight?
Before you hitch anything to your vehicle, check the owner's manual for the maximum towing capacity for your model. If you're not certain about what you're reading, call the vehicle manufacturer -- if you mess up this one thing, you're not only going to not get there, but you run the risk of getting injured or killed, and you'll definitely be out thousands of dollars of repair costs, especially if you were towing a larger toyhauler 5th wheel - for those can be quite pricy.
Third: Center The Weight Properly
Every vehicle that's designed for towing has a 'sweet spot' where the weight should be centered while towing. If your vehicle and load's combined weight is centered on that 'sweet spot', your tires will wear evenly and grip the road more effectively. If it's not, you'll get uneven wear and a slightly more dangerous drive. Follow the manufacturer's directions carefully.
Fourth: Special Considerations If You're Using a Gooseneck Adapter
If you have a normal kingpin hitch, you're done. If you're using a gooseneck adapter attached to a kingpin hitch, however, you've got one more thing to consider: extra force. When using a gooseneck adapter, the pivoting happens at the ball joint rather than at the kingpin. The two are usually close together, but the 'lever' action created by the adapter can actually put more stress on the kingpin than a normal kingpin hitch will. The end result is, you want to treat your kingpin's working load as though it were 3/4 or even 2/3 normal if you're using a gooseneck adapter to avoid excessive wear on the kingpin.