I'll just note that with your truck carrying an AF990 you are probably going to be significantly over the GVWR, and even threatening your rear axle rating, even before you start adding a horse trailer.
For comparison tanked up and loaded for camping with three of us in a DRW CC Gas 4x4 F350 carrying an AF1150 we weighted in at nearly 13,700 lbs.
Note the weight indicated on the drive axle! The AF1150 is only 450 or so lbs more in weight than the 990. Our truck weighs around 7,800 lbs. If yours is a gas regular cab it will weigh significantly less but if yours is a diesel CC it is likely to weight even more than our truck even though yours is a SRW.
In any case I suggest you stop by a scale and see where you stand.