Tags are a direct relation to weight. I started out only looking at Tags because I thought I would have a bigger carrying capacity, but in reality the tags just means you needed the 30,000 instead of 20,000 lb capacity in Engineering, usually because of >40ft. We ended up with a tag because we liked the floorplan with the bath and a half, but we lost some cargo space below because of the tag. Pick the floor plan you like, and whatever axles it has - it has. We looked at many without tags that were just as nice (some even one and a half bath), they just didn't need he extra axle based on weight.
With the Cummins, I was told to try to get into the ISL (8.9L). That's the 400/450 hp versions. Don't know why, just what my Cummins folks told me.
Fiberglass roofs are the best invention ever made! After decades of caring for those rubber roofs and peeling that lap sealant off of them, it was a real pleasure to go up on a roof with a scraper and roll off that lap sealant in a few seconds and apply new (I just added vent covers and Travler antenna on mine).