I am living your scenario as we speak, or type.
BEFORE I bought my 2025 310RL, I thought I asked the right questions with the right people. Problem was, most of those people were salesman. It didn't matter if it was at the GMC dealership or at any RV dealership, they all said the same thing. A 3500 with a Duramax can pull any RV on the lot with power to spare. And while that is a true statement, it will pull just about anything, it doesn't mean that it should.
Based on all that 'expert' information, I purchased a lightly used 2022 GMC 3500HD, Denali package, SRW, with the Duramax. Then I bought the RV a couple of months later and I had the dealership add a washer & dryer, slide toppers, and a Gen-Y hitch.
When I went to pick up the RV in Tennessee, the truck did sag a bit, but not too bad. I knew that I'd address that issue as soon as I could and I have already.
I pulled the RV back to the Dallas area with no issues. Truck pulled it like it was an 18' bass boat.
Then, I got to worrying and started to ask the questions on these forums' to the real experts. That's when I began to learn the truth.
In a nut shell. For a SRW truck, the number that you must watch is the RGAWR, and for my truck, fully loaded, that number is 7250 lbs. That is the max weight that your rear axle can handle. And it's not so much the axle that is the weak link, it is the springs, shackles, tires, etc., that will give way first.
I finally got my truck on the scales, then put both the truck and the loaded trailer on the scales. The actual weight on the rear drive axle is 7080 lbs. My truck had full tanks, me and my wife, and our jackets in the backset. The bed was empty, we had no 'cargo' to speak of in the truck and I even added some water to the fresh tank to help lighten the load up front on the trailer. As the math states, that gives me a whoppin' 170 lbs. of wiggle room on that rear axle. Just bouncing up and down hitting bumps in the road probably adds that or more momentarily.
Looking at your numbers you provided, you have 335 lbs. of cargo capacity, and you haven't loaded the trailer down yet, or put anything in the truck.
In hindsight, I may have gone with a dually. It would have given me the piece of mind to know that I could load the truck with a bunch of gear and still be okay on the rear axle, not to mention the added stability and braking a dually provides.
Since I'm not crazy about driving a dually all the time, I could have gotten a different package and not the Denali. My moon roof, electric steps, sliding rear window, etc., add a lot of weight to the truck. As does the diesel engine, but I'm not giving up that Duramax.
I am very conscientious of my weight distribution now and I realize I just can't load whatever I want without overloading that rear axle.
The moment of truth is when you put it on the scales. You can't put your trust in those 'advertised' numbers from the factory. The data stickers on the actual units, yes, but not the advertisement crap. For example, Alliance states the GVWR of the 310RL, on their website, is 15,500 lbs. In reality, the data sticker states 15,000 lbs. And I don't know where they get that advertised HITCH WEIGHT.
And speaking of the GVWR of the trailer. EVERY sales rep swore up and down that there was no way you could load that trailer down and get anywhere close to the GVWR. Granted, I did add some water to my tank, but my trailer, according to the scales, is sitting at 15,460 lbs. And that is, for us, fully loaded. We are not full timers, our storage bays and interior storage is not close to full.
Be aware of your numbers and even though you have a diesel and a HD truck, the reality is, the 310RL is at the top end for the truck.