packing a Fleetwood Niagara Highwall

That "dry" weight does not include any options lke the awning or A/C either. The high-wall Niagara will hold more (by volume) than a low-wall Niagara, so you are correct to be more concerned about exceeding the weight limit on the high-wall.
It dosen't hurt to record what you typically pack with a bathroom scale the next time you load up for a trip.
Liquids, canned goods, and coolers full of ice and food and are where we should focus on weight reduction.
There had been several posts in the past about bent axles on the HW - no doubt due to their ability to hold more stuff than they should carry.
There were some changes to the cargo capacity on the Highlanders over the years so keep that in mind too as you shop for a HW.

Having a similar floor plan highwall (HW) minus the slide, I can tell you my experience has been that I actually pack less than when I had a normal height PUP. It doesn't seem like that should be the case, but maybe it had everything to do with my method of moving my PUP gear from the old one to the new one:

Do not put it in the new PUP if I haven't actually used it, EVER, or even if not recently.

I ended up with a pile of gear in the corner of my garage that didn't make it into the HW.

Additionally, we decided to take advantage of more of the built-in items of the HW, so we actually use the inside stove for a fair amount of cooking, other than bacon and other meats - those still get, cooked outside on a single-burner Coleman (reduction in size and weight). We use the RV-Que that came with the HW and uses the existing LP tanks instead of bringing along a grill and a set of extra tank(s) for it, and so forth. It's small things that add up in big ways.

It really boils down to what you want to do and where do you want to do it. There are those who will never cook inside and will chose to bring along all that is required for outdoor cooking. That's fine. Everything is a trade-off. At this point, I'm opting for less stuff to have to manage and move around without really sacrificing much of the outdoor experience. We spend most of our time outdoors, eat outdoors and play outdoors, but we certainly have the benefit of managing less stuff and being able to fully utilize the equipment that comes with the PUP to its full potential, as we choose, and as others choose to or not to use.

One qualitative way I can measure this is the amount of stuff we place in the PUP's isle when towing. With the previous PUP, we used to fill it up full and hang the kids bikes off of some place on the TV or PUP.

Now, the isle is filled with the kids bikes, four camp chairs, an extra step stool because I did an axle over/under conversion and maybe the jerry can when we are not camping with water hookups. That's it.
The reasons for this are two-fold: 1) we have less stuff because we utilize more of the onboard systems, 2) with the extra length and height of this larger HW PUP, there is more onboard storage and while remaining true to #1, everything has a place that is out of the way. Thankfully, I have many compartments that are mostly empty and even one main deep drawer that is completely empty.

You most certainly can overload a PUP and especially a HW because of the extra storage afforded. Always give careful thought to what you put into a PUP for weight and usability reasons.

Thanks for the feedback guys...I really do appreciate it!

Whatever you get, we need pictures or it didn't happen.

I had one for 2 years. We were constantly battling bearing failures and uneven tire wear. My un-expert opinion was that we were having these issues because of weight. In the end we had to tow without water in the tank to stay under GVWR. I don't think we were serious over-packers, but YMMV.

Our 1850 lb. Aliner also doesn't have much cargo capacity, so we pack some of our stuff in the tow vehicle, so as not to overload the Aliner's axles. We have a Pathfinder, with 6000 lb. tow capacity, so we're good on that end. Not sure about your Explorer, especially if you're also loaded down with kids, etc. It's just my DH and me, plus 2 small dogs in our Pathfinder.