Gross vehicle weights and tires

Your tongue weight is leveraged by that long overhang, so the RV may not be as much over as you think. Most critical is the load on tires, as the axle has significant reserve and the rear springs are probably already supplemented by air springs on that year

If you have the OEM 225/75R16 load range E, your four rear tires can carry 9880 pounds at 80 PSI. Averaging over the four rear tires, you might be at most 5-7% over the tire ratings. That's within tire load safety margins, particularly in cool weather. Temperature is what makes a fabric body tire come apart (steel body plies are a different problem). Load, speed, road temperature all contribute to raising the tread temperature to the point of tread separation.

But that works both ways. The load and speed ratings are for a particular temperature, so that ytires can be "overloaded" on a very hot pavement even when loads are within tire ratings and tires properly inflated; that's when I slow down.

I am much more comfortable being 5-10% underloaded, than 5-10% overloaded, and would work to get those loads down. This is not easy on our max-size slideout carrying Winnebagos, I find managing weight a problem also with the shorter two-slide 29G, even though I don't put an additional weight on my bumper to load down the rear and lift the front.

Or I would look into one step up in tire size, 235/85R16 gets you to 10,592 pounds (XPS Rib). The higher profile keeps the dual spacing reasonable, although the larger diameter means higher effective gearing. Ford used to put this size on the SRW E-Series as an option, using the same wheels.

More extreme, an increase in wheel size so that I could upgrade to higher load range tires. 215/75R17.5 LRG will carry 11800 at 80 PSI, 12960 at 90 PSI, 14100 at 100 PSI. Those are for Michelin XZE2, probably the best multipurpose tire you can get for 17.5 wheels. You would be in trouble on axle and spring ratings long before you overloaded the tires.

The pressure you are running up front might accelerate tire wear there, because the OEM size carries maximum axle weight when inflated to 65 PSI. Much more pressure reduces the size of the contact patch to the center of the tread. I find 75-80 PSI changes front tire behavior enough to make the front end feel slippery.

whoah, you are overweight on the rear axle with a dually. You did say duals? I musta missed something. I guess you could go over on the axle,

.....

The axles will support 8900 lbs. or more. The 4 tires at the rear should be able to support 11,000 lbs. or more. Subtracting the weight of the truck with no load will give you an approximate payload capacity. The GVW is a number that indicates a class of truck for the feds, marketing, and the DMV. It is not a calculated value for your truck the way the "payload capacity" or GVWR numbers.

Even these numbers are calculated by the factory and based on the weakest link as the truck is configured when it leaves the factory. Nothing to keep you from upgrading the rear springs, shocks, rims, tires, adding a anti-sway bar, etc. to increase the safe payload for your truck.

The GVWR is also used to classify trucks and the big three all want to claim they sell more trucks then their competitors. It is why Ford even goes so far as to produce a pickup version of their F-450 and lumps its sales numbers in with those for the F-250 and F-350 pickup trucks.

Tell DW to get rid of her Rock Collection! :B

You need to provide tire specs and actual weight at each corner to answer several of your questions. It is also possible that all that overweight and more is on one set of dullies. (1200 over on one corner minus 800 under on the other still equals only 400 over on the axle!) And yes that would be a problem.
Traveling with empty or partial tanks will help dramatically.