You need to think about all the potential downsides you can think of before getting into this, especially if your towing and camping experience is limited.
Forget about "towing capacity". Payload capacity will be your "enemy" and you should look carefully at that. If the TV has factory options, the payload capacity will drop. The payload capacity is only for a skinny 145-150 lb driver and no other payload. The little tykes will add payload as they all grow taller and heavier by the day. Don't trust the payload capacity on the door pillar sticker. Before you spend $$ on a new TV, load it will all the passengers, and anything else you would include on a camping trip, fill the fuel tank up and take it to a scale and subtract that from the GVWR. Or allow for the weight of passengers and other stuff you may take in it. Take away another 100lbs for the WDH too. You *could* get a surprise - ours is 800lbs less than the sticker amount for example.
Forget about any dry weights for the TT. Unless you know 100% for certain what the actual tongue weight is, use 15% of the TT's GVWR. Some TTs run up around 15% and you could end up in BIG trouble if you guestimate the tongue weight. Same goes for buying a WDH in advance of taking it to a scale.
For a family that size, you could have above average payload in the TT for all the clothes, food, toys & games, and other supplies. And if you had one or more full holding tanks, you could end up well over the GVWR. With our TT, we'll be over the GVWR with just one full tank and we only have stuff in the TT for two people and short trips. More TT weight = more payload on the TV's hitch.
39' is a long, long trailer. Think of what it will be like the first time you are on the road and all the mouths need feeding and you try and find a McDonald's to pull into. Or try finding a gas station to get that monster into. Same with stopping at a supermarket somewhere to get groceries. I find it hard enough sometimes with just a 29' TT. With a longer wheelbase TV and 39' TT, you will find it a challenge sometimes to get around corners.
Kids love their own independence and they love being able to sleep in a separate tent (unless they're really young). You could find that a smaller TT with a tent or two might be an option. If it's cold or raining, then it's closer family time... Have a good TV setup and stock up on lots of DVDs. If it will fit, you can replace a jackknife sofa with a hide-a-bed and gain more sleeping space.
Even if the TV could handle it on towing capacity, you don't want to be towing long distances and/or going up lots of hills near the max. rating as it will be harder on the engine and drivetrain and harder on braking. I would get the TV and TT brakes serviced to make sure they are in top condition. A good brake controller is a must too.
Lastly, before you sink a lot of money into that TT, I would look it over very carefully and especially anything to do with the frame and suspension. I would even suggest having an inspection done by a trained RV tech. If it were me, I'd even take it to a frame and axle shop to get the frame looked at. You never know what is lurking somewhere in or under that TT. Even brand new units can have major issues that may not be found by the average RV owner. The last thing you want to have to do is spend a lot more $$ on it for repairs or be stuck out on the road somewhere with a breakdown. Water damage can kill an otherwise good unit and you may not see clues at first.
When you get it all figured out, have fun camping. :)
