Due to the different characteristics of a fifth wheel trailer, this applies more to a trailer than a fifth wheel.
Why is length such an important factor? Well, it's not really the length of the trailer that is as important as is the size (or wheelbase) of the tow vehicle trying to pull it. The main focus of this is to minimize trailer sway, which in many cases is caused by the wind from either Mother Nature or large vehicles passing you by.
Basically, the longer the wheelbase the better! Think of it as leverage. The longer the trailer, the more leverage it can have on the tow vehicle. The longer the wheelbase of the tow vehicle, the more it can resist the leverage being applied from the trailer. You don't need a crew cab long bed truck to pull a pop-up that could easily be towed by a small SUV. But you don't want to pull a 30' trailer behind that small SUV. You want something longer. But don't get carried away, either. Let's see how it works.
You'll need 2 measurements, the wheelbase of your tow vehicle, and the total length of the trailer you are pulling (or intend to pull). That length is from the coupler to the back bumper.
The First Guideline
(This guideline was first used by the RV Consumers Group rv.org)
For the first 110" of wheelbase, this allows you 20' of trailer.
For each additional 4" of wheelbase, this gets you 1' more of trailer.
Wheelbase / Trailer length
110" = 20'
114" = 21'
118" = 22'
and so on
The Second Guideline
The distance from the coupler to the rear trailer axle should be no more than twice the wheelbase of the tow vehicle.
Why this second guideline? I think this second guideline applies more to non-RV trailers, like boat or flatbed trailers with short (less height) loads on them. Those types of trailers usually do not have the large vertical surfaces (exterior walls) that would be susceptible to wind. You've probably seen a small SUV or pickup pulling a very large boat, right?
If you look at some of the physics and geometry inherent to travel trailers, you might see why length is an important factor to consider. Ever try to carry a full sheet of plywood (or something similar in size) by yourself, on a windy day? It can be pretty difficult to maintain control. But how about carrying a couple of 2 by 4's on that same windy day? Not so hard, is it? That's because the 2 by 4's do not have the same surface area to catch the wind as the sheet of plywood does. So, in a way, that travel trailer is just like a sheet of plywood for catching the wind.
The next thing to look at is how far the coupler is from the trailer tires? The greater the distance, the lesser the impact it will have on the tow vehicle and the less sway it could create. You will see travel trailers of the same overall length with the axles in different locations. This is probably due to the floor plan or layout of the trailer in order to balance the overall trailer, as well as to provide enough, but not too much, tongue weight.
Finally, the ball, or hitch location. How far is it from the tow vehicle's rear axle? The farther away it is (known as rear overhang), the more leverage the trailer can apply to the tow vehicle and create the possibility for more sway. A Jeep or a Hummer would make great tow vehicles because they have very little rear overhang compared to most pickup trucks and SUV's. Another example of this would be in comparing a 2003 Chevy Tahoe to a 2003 Chevy Suburban. The wheelbase of the two are different, but their wheelbase to rear overhang ratios are almost identical. Therefore, both vehicles should be able to handle the same trailer with similar results as far as length is concerned.
If you end up breaking the length guideline by a few feet (3'-4'), you might be okay as long as you have a good sway control hitch or anti-sway bar. Usually, the only way you can tell for sure is taking it for a tow and see how it handles. You may be able to get the RV dealer to let you take it for a "test tow". If you do, keep in mind that the trailer is pretty empty and does not weigh what it will once you pack your stuff in it. If you do decide to purchase based on a successful test tow, make sure you have adequate tongue weight once you are loaded for the road.
If you end up breaking the length guideline by more than a few feet (3'-4'), but are under your weight limits, consider looking at a couple of products that may help this situation. They are the Hensley Arrow coupler, the ProPride 3P and the Pull-rite hitch. Both of these products, as well as their owners, say they do a great job at what they are designed to do, reducing or eliminating sway. So check them out and see what you think.
(The references to the Hensley Arrow and Pull-rite products are not intended as an endorsement or advertisement of either product.)
Our first trailer was a 2001 Prowler 27H (actually 28' long) and I pulled it with a 2003 3/4 ton Suburban using a Eaz-lift hitch setup and a friction anti-sway bar. I was over the first guideline by about 3' and just under the second guideline. Now I'm pulling a 2008 Adirondack 31RL that is 34' long, and I upgraded to a Reese Dual-cam setup. This puts me 9' over the first guideline and just a couple feet over the second. I've never had any serious problems with either setup. The only time I notice any wind is when the big rigs pass me by and the trailer will wiggle a bit, but nothing serious.
Keep in mind that these are just guidelines. There are many factors that could make one combination of tow vehicle and trailer safer or more stable than another, such as the amount of rear overhang, hitch to trailer axle length, hitch setup, load displacement, weather and road conditions, just to name a few.