Hi Lane Hog,
If your E450 is long and heavy, you owe it to yourself to consider "E" rated tires that handle an extra 500-600 pounds per tire. There are two types of these extra load tires. One requires 83 psi to get the full rating, the other requires 90 psi. Get the 83 psi because the E450 stock steel wheels are rated for 80 psi.
CLICK HERE to see one such example, but there are different brands to choose from.
If I owned a long 28-32 foot class C motor home and needed new tires, I would surely get the extra load tires to avoid a tire blow-out, a common problem with the largest and heaviest class C's.