Disadvantages of running a 65PSI vs 80PSI E Tire?

Do you know the actual weight on your rear axle? My setup in the sig line is somewhat similar to yours. I am within a few 100 lbs of my RAWR even with this fairly light camper. Just make sure real-life weights are within the tire's capabilities.

I did some googleing on the subject and came up with the observations mentioned by other posters above, namely a more "mushy" feeling under load.
https://www.google.com/search?q=some+load+range+e+tires+are+65+psi+max

Jimh is the only one that has given you accurate info so far.
There is no disadvantage to 65psi, the max pressure rating IS 65 psi and the load rating is correct. Tire is wide and tall enough that, that pressure gives it that full load rating and the wider and taller the tire, generally the more load it holds for a given pressure and the wider the tire, the more it balloons at higher pressures. And no it won't feel squishy compared to 80 psi in a narrower tire.

This is consistent pretty much across the board for tire ratings.
Never had a set but they look awesome. Huge tread depth and always thought Yokos were top quality. Curious what Spectra doesn't like about them.

Only comment is if you're using them in the upper end of their rated capacity and have OE rims, 8" rims are a bit narrow for that tire. Not overly narrow but not optimal for best tire wear. Ideally 9-10" wide rims with 12" wide tires.

In the world of load carrying tires 65 psi is pretty soft and can lead to carcass roll on tall tires with tall sidewalls. Thats been my experience anyway when using over size tires and high hitch loads from heavy GN flatdeck trailers.
For your F350 and the heavy truck camper I would stick with a 80 psi rated tire

I’m assuming the question is what is better if both have the same capacity, but one is 65 and one is 80. My guess is that the tire with 80 will ride rougher and have slightly less sway due to having more air in the tire. However, it could work out different if the reason why the tire is 65 and the same load capacity is that it has a stiffer sidewall.

I do know that the same tire with 65 and 80 will ride softer at 65. I’ve tried that.

65 vs 80 PSI depends largely on the load on the tires. My SRW 18” Tire’s with camper on are set at 80 psi and my camper is heavy. Really heavy, I’m over weight on tires and axle, so I run them at max pressure and watch the TPMS like a hawk and feel the tires at every stop to check for excessive heat. Heat is a big concern when you start lowering tire pressures because lower pressures add flex to the sidewalks and thereby adding heat. To much heat can cause a tire to fail. Your best bet is to weigh your truck and see how much weight on the tires and determine if can safely run at 65 psi.

I had a set of Geolander ATs on my truck. I wont buy another set of Yokohama tires.

Sorry I can't help you.

Depends on the load you want to carry

I’d pick the tire with the most weight capacity.