Rob, Gross trailer Weight (the weight of the trailer plus all the stuff you carry inside it )
Tongue weight (TW) , usually 10-15% of the gross trailer weight can cause the rear of the truck to drop and the travel trailer (TT) nose to drop . This will remove weight from the front wheels of the tow vehicle (TV) . Traction and steering of the TV will be affected.
If the TT has too little TW , this can cause trailer sway where the TT swings back and forth. These two factors is the reason to use a weight distributing hitch (WDH).
Generally speaking,the WDH puts back that weight onto the TT and trucks axles and wheels and the combo will appear to be back to level.
The spring bars used have to be close enough to match the real scaled TW of your loaded up TT. Having a little extra spring bar rating on the WDH is not going to hurt you. (ex: if your real TW is 950#, a 1200# spring bar will work fine as will a 1000# bar, but i would prefer the extra 1200# bar because your TW will shift depending how much stuff you carry and how you pack the TT)
The times when you would NOT need a WDH is when your TV is stout enough and the TT is light enough that the TW does not have any impact on the truck. Most people towing TT's with 1500 trucks and SUV's usually have to use a WDH due to TT weight .
You can buy a SHERLINE TW scale $140, go to a public scale or use the bathroom scale method (google it) to measure your real TW which might be double your dry TW. (dont be shocked :E)
You need to know the real scale weights of both TT and TV all loaded up for a trip. SET up your hitch properly. All you need are sockets and a tape measure.
Do it yourself and don't allow a dealer to do it for you. RV dealers have a broad range of talent and for some reason sometimes they allow the less talented to set up hitches.
It will take a few hours time but its worth it, do it right and your towing experience will be a good one. Your TV should feel like it's in control of the TT. If not, you need to fine tune the WDH.
I use the following WDH set up because its a universal set up that covers the basics. You can also use your specific WDH mfg suggestions for fine tuning the set up. But get the basics down first.
WDH set up .