I have a lot of experience with my first generation LB7 pickup pulling our toy hauler trailer (both gone now in favor of the Seneca 34SS). I'll try to encapsulate my experience and it may help. Your Seneca is likely on a 2006 Kodiak chassis, which is likely one Duramax generation after mine. But they are so close, I think this will apply to yours. First, a few axioms that will help: 1) Your best fuel mileage and most comfortable rpm range for your engine is the one that yields the greatest torque values. In these engines, it's around 1600-2300 rpm, rounded to 2000 rpm. 2) Your engine (and especially your turbocharger) despises being lugged (high torque requirement at very low rpm) even though it is a diesel. It would rather rev a bit higher. 3) The MH version of the Allison doesn't have a "tow-haul" mode. It's always in "tow-haul" 100% of the time. 4) Your engine and trans will be happiest when you are climbing any hill in the right rpm range without having your foot on the floor. 5) Your trans is happiest when you are not constantly hunting from gear to gear, or especially from overdrive to 4th/5th/whatever is right below OD on your particular version of the Allison. 6) Your turbo is happiest when it is spinning fast at low throttle settings like 1/2 to 3/4 throttle. 7) The cases are rare when you want to have your foot on the floor with the loud pedal for any extended period of time. Like when you are pulling up an extremely steep hill in 1st gear and barely able to keep it moving. As I mentioned, that's rare.
OK. So, with those in mind, here is what I do when climbing hills. Essentially, you want to be in the highest gear that will allow you to sustain approx 2000-2300 rpm without using more than about 3/4 throttle. That may mean you want to slow down on the ascent well below the speed limit. The Duramax is a wonderful diesel engine, but it does have its limitations. 325 Hp is not going to haul 20,000+ lbs up steep hills in overdrive very often. Chances are, you'll be downshifting. When that is necessary, I will take the extra step of ensuring it won't try to jump back up a gear when rpm is higher than 2300 or 2500. At some point the trans senses rpm, throttle position, and make a decision that the next higher gear is appropriate, and you'll find yourself bouncing from one gear to another (refer to axiom #5 above). As soon as it jumps down out of overdrive, I lock out overdrive. The moment it drops down from 4th to 3rd, I lock it into 3rd, and so on, if the hill gets steeper and longer, until it can sustain 2000-2300 rpm easily around 3/4 throttle or less.
You'll find that higher rpm in very hot temps will begin to heat up the engine cooling system. It's also heating up the oil. Following close behind will be the trans temp, which will start to climb. I never let the trans get into the "yellow" warning zone for temp. I never let the engine get beyond about 230 degrees (you'll likely have to estimate where this is on your gauge in relation to the 210 mark). If necessary, I drop down a gear and lower the rpm to 2000, and the temp will likely begin to drop back down. INcreasing heat is an indication you are overloading the engine and/or trans. The engine coolant will drop in temp much faster than the trans will. This is likely to show up only on very warm days, assuming your cooling systems are in good shape.
You will not likely encounter hill as steep or as long as the ones I encountered in Colorado last week. My Seneca performed very well there at high altitude, despite the lower density air, which decreases cooling capacity. On that trip, my only potential issue was on a long uphill on I-40 near Kingman Az, on the way to Colorado, where there was ongoing road work to replace some bridges and repave a portion of the highway. There was about a two hour delay in bumper to bumper traffic at about 100 degrees F while climbing and holding on a steep grade. Motorhome is not moving forward as much as 2 mph in stop and go, so no air flow through the radiator(s), a slight breeze was at our back, it was working hard in low gear (stop and go on a steep hill is really tough on these heavy vehicles), and heat is radiating off the pavement and adding to the engine heat.
As I crest a hill after working the drivetrain hard coming up to the top, I begin to go back up toward high gear one gear at a time with a moderate throttle setting 1/2 to 3/4 throttle. Keeps the coolant and trans fluid flowing. I use the exhaust brake almost all the time on the highway, and just keep it in whatever gear will maintain the downhill speed I want.
Your vehicle is likely around 20,000 lbs empty, and I don't know what your loaded weight is with any toad you may have. Mine is about 25,000 fully loaded with a Jeep toad. That's a lot of weight to be pushing/pulling up hills. Won't likely be too bad around San Diego. A bit more in Arizona. But once you get up on the plateau, it's pretty small hills mostly. Just don't get in too much of a hurry, and you'll be fine.
I hope this was helpful.
2006 Jayco Seneca 34SS
2006 Jeep Wrangler Unlimited Rubicon Toad