I see you are past your axle install reading this post. It appears the shop used a premade axle (or selected the wrong size,) and not one to match your camper. This is not the end of the world, just a realization of what they did.
I'll explain a little what you may have seen when you said this.
The hub to hub as measured by tire center to tire center on the old front measured 83 3/4, now measures 84 1/4. The rear measures 83 1/4. The installed says put on a Dexter 84 and it is marked as such. He also assured me that the axle was replaced at the same spring attachmet as the old.
The front I'm assuming is the bent axle. And you stated you measured the "tire" at the center and on the old got 83 3/4" on the front and the rear axle is at 83 1/4".
When doing axle work on bent axles, measuring to the tire center is not a good way to determine hub face to hub face. The hub face is the brake drum. The axles even when new can have a slight bend in them to create what we call "toe" angle. Ideally the tow angle is 0 degrees, but it can have some small tolerance of toe in or toe out, meaning the wheel points inward or outward at the front of the tire. 0 deg toe the tire is dead on straight ahead.
With a bent axle stub, the toe angle is all messed up, this is what is wearing your tires out so fast, the tire is at an angle to the road. It is scrubbing hard all the time and wears faster then a tire rolling dead true straight ahead.
Since the OD of the tire is very large compared to the OD of the machined surface of the brake drum, that large error on a bent axle will measure wrong on the hub face dimension at the center of the tire, especially if the front axle has excessive toe out. The tire is pointing outward in relation to the center of the trailer.
Most likely your axle was a 83" hub face axle at the brake drum as the rear axle with the 83 1/4" at the tire reflects this. I'm just mentioning this so you understand how this all came about.
So now you have an 84" hub face axle, but the shop stated the spring center distance is the same as your old one. The spring distance is very important it cannot tolerate being a 1/2" off. And if it was, the spring would have a real hard time to even fit in the spring hangers properly.
OK so is the tire being out 1/2" a problem? It's not great but it may not affect anything as long as the tire does not rub the wheel well in a hard 90 degree turn of the camper. I'm not sure what your vintage fender wheels look like, they may be wide open at the top of the tire and if so, then there is nothing to run into. Just check this fender well area for tire or fender rub marks.
As long as there are no tire rub marks from the fenders, the trailer will track correct with the tire being 1/2" sticking out further.
You mentioned you had tire failures in the past, you have all new tires and you are heading out on a long trip. I'll pass on these comments in case you did not know.
1. When the tires are changed on and trailer (camper) axle, front or rear, the lug nuts have to be torqued (100 ft lb) for 1/2"-20 fine threads wheel studs at least three times in sequence once the tire is mounted. The mile recommendation will vary, it is like the first 50 miles, then 100 miles, then 150 miles with the first being the most important. This means you need to have a 1/2" drive torque wrench, a 13/16" socket and an extension to check the wheel torque during this settling in period. The issue can/will be, until the lug nuts have settled in, and some times you need to go 4 times to checked until they stop taking any little extra torque on a recheck with the same torque wrench. If this torqueing sequence is not done, the lug nuts can get loose and left unchecked, it can create a problem. If you have new painted steel rims, the issue is worse then older steel rims as the paint is being squeezed out of the bevel in the wheel. This torque wrench setup (a 1/2" drive one) can be a simple one from Harbor Freight and does not cost much. They are good enough for trailer wheels.
2. Look on the side of your tires, since you have two, 3,500# axles, they should have tires on them where each tire has a weight rating of 1,750# or higher. The 93 Sunline brochure calls out 14" tires, which would be ST205/75R14 Load range C at a rating of 1,760# each tire. And the tire calls out 50 psi max cold side wall pressure, again molded into the tire side wall.
What tire size and load range do you have? The tire shop may have installed a larger rated tire, a load range D in place of C, which is 65 psi. This is OK as the rating is one size up and higher.
Point: The tires need to be sized to handle the full 1,750# and the tire needs to be aired up to max cold side wall pressure at the "start" of each towing day. Under inflation is a leading cause of tire failure, next is undersized.
3. Many ST trailer tires have a max speed limit, many with a max of 65 mph. Some of the newer tires are rated higher, but you need to know what yours are. It is not advised to tow at speed over the max speed limit for extended time periods, the excess heat build up degrades the tire faster. Going a minute or two to pass some one is not the same as towing over the rating hours on end. As FYI, I only tow at 60 mph as a rule on the interstate. My tires and truck can do lot more, but things can go real wrong, real fast when you start getting to 65 and above mph and something starts to happen to upset the towing rig. That and the fact the fuel mileage goes real high, real quick above 60 mph.
Hope this helps, have a great time and post some pics of your journey. It is always good to see Sunline in action out camping.
Feel free to ask for any explanation on any of this. Glad to help.