I'm looking for a similar travel trailer (29') and about 6000 lbs . I want to travel long distance with it. Is the size cause any problems or less handling ? At the same weight does a 24' have a better handling than a 29 ' ? And finaly does a sway bar is mendatory ?
How are you determining the weight of the trailer? If looking at the dry weight then that will never happen, it will always weigh more. If you want to know if you could POTENTIALLY tow a given trailer then you should make sure the GVWR listed on the trailer's placard is less than your truck's max tow rating. That will at least get you in the ballpark.
As for length, yes that will make a big difference in how it tows. The longer the trailer the more surface area on the side, the more trailer is behind the trailer's axles, and the more leverage there is placed on your truck by wind pushing on the side of the trailer as well as weight behind the axles acting like a pendulum. The term "tail wagging the dog" is common when a trailer is too long for the vehicle that is towing it. Honestly, I wouldn't tow a travel trailer longer than 24 or 25 feet long with these trucks. 29 feet is likely going to be unpleasant, especially when dealing with crosswinds. A shorter trailer will definitely handle/tow better.
A weight-distributing hitch is required and you would want to use the best sway control hitch you can afford. I would recommend the Equal-I-Zer hitch, best sway control for the money.
But, let's get back to how much weight you can REALLY tow. Open your driver's door and look at the B-pillar (the post where the seat belt mounts), there will be a sticker that tells you what your max payload is for your truck. Take that number and subtract the weight of yourself, any other passengers, and any gear you've put into the truck. Then subtract another 75lb for the weight distributing hitch gear. That is roughly how much weight you have left for trailer tongue weight.
So let's say you have 1,300lb payload capacity. You and a passenger weigh 450lb combined, you don't have any other gear in the truck, and you have the 75lb hitch gear. 1,300 minus 525 is 775lb of payload capacity left for hitch weight. The hitch needs to be between 10% and 15% of the trailer's actual weight, 12% is a good number to shoot for.
So 775 / 12 * 100 = 6,458lb MAX trailer weight. If the dry weight of the trailer is listed as 6,000lb then by the time you add 2 propane tanks, a battery, food, bedding, clothes, chairs, and whatever else you need for your trip then you will be overweight.
I'm sure this sounds complicated and it can be if you're new to towing and travel trailers. Salespeople don't care about reality, they care about sales. They will quote dry weights for trailers and MAX tow ratings for trucks and then get you to believe that your truck can tow a given trailer NO problem. That has got a lot of people into trouble by being overweight.
Find out what your REAL maximum tow rating is for your actual truck and then find a trailer that has a GVWR that is no more than that weight. That is really the only way to help ensure you don't end up having too much trailer for your truck.