I've been towing a 26' Outback travel trailer with the rear slide (like yours) for almost 6 years now. The 'scaled' weight (very little water in the fresh water tank) was 5,600 lbs. This was with all the stuff in the trailer before heading out on a camping trip. Before I put on a 6" RCD lift and re-geared, I had stock gearing (with tow package) and it towed just fine. We are a family of four and I toss in a Yamaha 3500 watt generator for those trips that need it. I certainly know the trailer is back there and I drive conservatively while towing. Keeping extra distance between us and the vehicle in front. I stongly recommend getting your truck and trailer weighed, then you will know exactly how much it weighs, don't rely on the stickers and guesstimates.
I too have a Prodigy brake controller and W.D bars. With W.D bars hooked up, a very general rule of thumb is to aim for having your truck and trailer level. This is accomplished by adjusting the chains at the end of the bars. Typically referred to as the number of links 'hanging'. I have 5 links hanging or dangling free on my setup. The more links hanging, the more tension is applied to the bars and more down force on your truck. It is important you understand this relationship. I would suggest taking your truck and trailer out to a nice flat location, along with a measuring tape. Unhook your truck from the trailer, then measure all four corners of your truck and jot it down. Typically the distance from the ground up to the top of a wheel well. Now hook up your trailer with the W.D bars and measure the truck again. If the bars are adjusted correctly, the truck should 'drop' an equal amount on all four corners, say 1 inch as an example. If the rear of the truck drops more than the front, then not enough weight is being transfered via the bars to the front of the truck. So, increase the number of links hanging and remeasure. An excellent mod to your trailer is an Atwood power jack. After doing the above process a few times to dial in your W.D bars, you'll see why!
If you are concerned about long grades, I would suggest installing a transmission temperature gauge. The BEST location for the sending unit (translates the temp of the oil to the gauge readout) is in the tranny pan. But that takes some work. I bought a second pan, had a friend weld in a bung for the sending unit, then swapped the pans when I had the time and wired it all up. I bought the gauge and A-pillar pod from Summit Racing (pics in my gallery). The other alternative is to install the sending unit in the rubber line that runs just below the radiator. Just remember to use a ground wire attached to the sending unit or it won't work. This is easier than the tranny pan, but the engine compartment heat will affect the readings, especially in stop and go traffic or long climbs, otherwise it works pretty well. For added protection I bought a larger tranny cooler from these guys:
IPT Performance. Scroll all the way down to the bottom section titled 'Other for tranny coolers. They also sell an Autometer tranny temp gauge and sending block. I also bought and installed an in-line filter. All of this is on the same web page.
From a safety perspective, I would highly recommend one of these:
Tire Pressure Monitoring Systems by Doran. With a dual axle trailer, you may not know you have a problem with a tire until it's too late! I've been using one for about 3 years now and they work great!
I've towed our Outback from where I live in California to Yellowstone, the coast of Oregon and California, central Oregon, Yosemite, Death Valley, southern California, and Nevada. The hottest temp I've ever seen on the tranny temp gauge was pulling a 9% grade near Death Valley, it hit 250 degrees. I believe a tranny temp warning light comes on at 270. I'm also running Amsoil synthetic tranny fluid. My Tundra now has 126,000 miles on her and I have a ten day camping trip planned for June that will take us up to Mt. St. Helens and Mt. Rainer in Washington, then over to the Washington and Oregon coasts for the return leg. Overall, I'm very happy with my setup.
Good luck, get camping and feel free to ask more questions as they come up. It is much better to ask than not.
