Well, I have done a lot of reading and I wouldn't be surprised if I have a few nay sayers pipe up, but any feedback, opinions, or experience welcome. I did a lot of wrench turning on my truck to install modifications to help make the truck reliable while towing at these weights. I also should note that I installed a set of Comfortride shocks on the trailer. This feedback is related to our first trip from Oceanside, CA up to Sequioa Natl Forest and back. I have lots of feedback and I scaled the truck and trailer. Here are some legit numbers:

GVWR: 25,300 (25,200 max) (full tank and everything packed and rolling to include all occupants)
Truck weight:
Unladen: 8,900 (includes full tank, generator, 5 gal of gas, me, wife, dog, Superlite hitch)
Laden: 12,060 (12,500 max)
*NOTE: No emissions stuff! (DEF tank, bracket, EGR, exhaust, grid delete, etc)*
Rear axle weight:
Unladen: 3,260
Loaded: 6,880 (7,000 max)
Trailer: 2022 Grand Design 390RK, 41', 5" long
Trailer weight: 16,500 (16,800 max) (Propane tanks full, 5 gal propane tank, fire pit, grill, mtn bikes, and baggage compartments barely used)
*We took a lot of stuff out like canned goods, glass items, pots/pans, clothes, etc)
Pin weight: 3,250 which is about 20.5% on the truck.
Hitch: Pullrite Suprelite. Hooking up and disconnecting is super easy. I do use chains as a safety. I did note the pin plate warped under the stress of the Pullrite hitch setup and we have ordered the reenforced plate.
Trailer Brake setup: Heavy electric
Gain heading up: 8.0 worked well and was appreciated during a panic stop that locked up the front tires briefly. But, they were very grabby and annoying in stop-n-go traffic and city areas.
Gain heading back: 7.0 worked better and were less grabby and annoying. Might bump it down to 6.5 next trip.
MM3 Setting:
Heading up: 30 HP setting. It was kind of a dog and really struggled up grades. It was fine on the flats and downhill. The EGT was in and out so I dont have feedback under all situations. It hit a high of 1,100 up grades.
Heading back: 60 HP setting. We took a different route and hit some traffic. It was much better on the grades and accelerated with more confidence. Again, the EGT was in and out. On one particular climb it hit 1,194 degrees but was stable until it quit working.
Fuel mileage: 9.5 hand calculated (EVIC 10.5). This was less than we expected, but we had a lot of grades, traffic, winds, and idle time that contributed to this.

Feedback in several areas:
-Overall driving experience: I found it to be quite manageable. I set the cruise on 67 mph (when I could) and locked out 6th. We took the 5 north through LA traffic which turned a 5 hour trip into a 7+ hour trip. The trailer brakes were pretty grabby in traffic and I did have one panic stop where my wife and I were crapping our pants since we had no where to go, but the truck hauled us to a near halt with distance to spare from 60ish. The truck struggled up grades but still maintained speed up hill after a downshift or two. The transmission shifted more promptly (Installed REVMAX billet valve body) but had multiple occasions of bang shifts between 3-4 and a few 2-3. On the way back we took some back roads and skirted the bigger mountain range and headed out into the desert area and took the 15 back. I tried to use 6th for a bit. The truck would hold the gear with the Billet plate and tune, but it hunted on any inclines so I locked out 6th. Out here we experienced 25-30 mph sustained winds with gusts up to 35 mph from our immediate right (90* from the truck). The truck was busy but I never felt unsafe. The biggest wind effects were passing semis, underpasses, or terrain that blocked the wind and then got hit by the gust on the other side. They required more correction and certainly felt less comfortable. The truck ran better on the 60 hp setting overall but I would caution folks on using this while pulling up grades (where it was the biggest benefit).

We really managed our loading on the truck and trailer and found we didnt need anything we left behind. We just stuffed it all in my Fusion and the wife's Grand Cherokee. As you can see by the numbers, we got lucky and were at the max for everything. I dont know that we can take much more out unless we start sacrificing a few things. Next steps are a regear to 4.10 gears and a step up to a 295/70R18 to gain some leeway in our reality numbers (not our sticker numbers). Hopefully with that we can pull on the 60 hp setting and use 6th while being able to get this beast moving easier.

My truck was well matched for out GD 315 RLTS which was nearly 38' bumper pull that was about 12,000 rolling. My truck pulled that with ease and I could even use 6th and pull at 75 without too much concern. This bigger 5th tested the combination more for sure but was still withing the limit

We have future plans to upgrade both the trailer axles and the truck to a dually. For now, we are just trying to get close to my retirement (2-5 yrs).