#1 - I guess it depends on how many windows your new 5th wheel has. Having owned both types, I'd never be without double pane windows if you're going to be camping in colder or hotter climates. They also offer substantial sound deadening benefits.
#2 - Since you have a short bed truck, be sure you don't need a sliding hitch with the new 5th wheel before you change hitches. I use a B&W RVK3600 for my Ram that fits in the factory pucks, but with a long bed truck I don't need a slider.
#3 - I use Lynx Levelers. I carry 2 bags of them. If the site is fairly level, I'll use 2 levelers under each pad. If not, I'll move some from the high side to the low side and/or use more.
Rusty