Welcome! First rule about towing with a half ton: forget about the tow rating.
Hear me out:
Especially if you are traveling with more than just yourself inside the vehicle, you'll always exceed the payload rating well before reaching the tow rating.
First rule about payload: forget about any numbers you found online and go by the yellow sticker on your truck.
So, here's the simple anecdotal view:
You'll be fine, lots of people have towed uhaul trailers all over this great country without ever so much as considering your questions. Asking shows you are well ahead of them. The Uhaul trailers are overbuilt and designed to carry weight pretty well balanced.
Load the Mazda engine first about as far forward as possible, and load any cargo you can into the Mazda, not the truck. Keep a keen sense for any wobble or sway when you are driving. If detected, a bit of braking can sometimes reign it in. If it doesn't, pull over and see if you can shift weight forward on the trailer and/or in the Mazda.
And here's the detailed numbers-obsessed view:
Here it's all about Payload; yours specifically @ 1,245. This is how much weight you can carry before anyone gets into the vehicle, or you put so much as a twikie in the glove box. Any and everything in or attached to the truck deducts from there.
Let's start our deductions with your trailer:
- Combined weight of Uhaul + Mazda + cargo = 6,085 lbs
- Add 10% unless you've taken this combo to a CAT scale, because the 2nd rule of towing is that it always weighs more than you think = 6,694 lbs.
- Determine Tongue (hitch) weight; this is what will be deducted from your payload. TW is usually around 10%-12% for a bumper pull. 11% * 6,694 = 736 lbs.
- Deduct tonue weight from payload: 1,245 - 736 = 509 lbs
- You now have 509 lbs of capacity left for any people, cargo, and hitches in or on the truck. You'll have to take it from here to get your exact numbers, but if you can ensure that less weight than that will go into the truck, you will be compliant with the payload specs while towing.
- You can see why its best to load any cargo you can in the trailer vs. in the truck; it only penalizes you 10-12% as much. But don't put any humans in the Mazda, unless they really, really annoy you.
- If you are a little over payload, I'd say you're fine as long as its properly loaded as above & pay attention to sway as you drive. Better to have a "non-compliant" trailer driven safely than the invese, IMHO. Others may mention liability issues if you were to cause an accident. Not me though. Nope.