tires again... two sizes, unusual but does it matter?

Personally I want all the tires to be the same size. Tire issues are one of the most frequent problems most RVers have to deal with. If finanically possible, I would also switch to 15 inch rims with "E" rated tires. For me, I would rather have a little over kill margin of safety as opposed to using tires that are at their maximum capacity of weight load.