The manual/Isuzu will NEVER give any recommendations on air pressure other than OEM.
Depends partly on what specific tire you actually get, have to look and see what it's max tire pressure is, as listed on the sidewall, then go from there.
Both 285/60-18 Cooper Zeon LTZs and Nitto TerraGrapplers have a max of 50 psi listed. Since I regularly drive on rocky/dirt roads, even 44 was just too rough and jarring for my liking, so I choose to run a little lower...front 41, rear 40 psi. Just an FYI, you'll get a bit higher MPG the higher pressure you run (but of course, don't exceed the max recommended).