None of the P chassis coaches had the 8.1L engine with the Allison transmission. For that combo you have to go to a coach with the heavier W series chassis. The 8,1L engine was used on the later P series chassis but it was coupled with the GM transmission, which is a fine transmission but it is not the Allison.
As with any coach, check the unloaded weight and the maximum gross weight and see if that coach will allow you to load the coach the way you would travel. The P series chassis are usually rated several thousand pounds lighter than the W chassis.