The tag gives stability and, as you mentioned, it allows for more cargo carrying capacity. I don't see an issue with 'scrubbing' tag tires as they will age-out before they would ever wear-out.
Most tags will lift for tight turning. Ours will 'release', it lifts but does carry some weight. At 30,000 miles, we have very little wear on the tag tires.
Newell coaches offer steerable tags and maybe some others.
A 40 ft. coach should have a 400 hp or more. A Cummins or Cat shouldn't make any difference, they are both good engines. CAT has not made commercial over-the-road engines since 2007 or so.
Good luck,
MM.