The LT tyre isn't all that crucial with duals but with a single axle you really need the LT tyre for the load rating of 110 or higher just to get that margin, you don't need the OffRoad tyre, just need the Load rating to suit your weight plus a bit more, off road tyres do look cool though but an aggressive tread pattern doesn't do a thing, it's not a driven wheel, a good ATR or trailer pattern is better.
See Bob Jane's
http://www.bobjane.com.au/info/load-index-speed-symbol/ it's a handy reference also
http://www.carbibles.com/tyre_bible.html it will give you the difference for various wheel/tyre profiles, so if you go from 15"to 16" just change the profile and you won't throw your height out too much, you can mix and match to suit. For a change later on it is crucial that you make sure you have room in your guard for the tyre and thats allowing for spring compression, there's a lot of measuring from all perspectives, it's not just does it fit on the rim.
With 15" you should be able to get a 104 rating off the shelf, above that seems to be an order in with the 16"the rating is higher for off the shelf stuff. But then if you want the look at me tyre you better go 16".
With radials the ply ratings are not all that differn't where with the old rag tyre you could shop around for an 8 ply and it actually did have a thicker wall not so nowadays.
As for compliance plate, well since Jayco issue plates with the wrong info, really it's not much of an issue, nobody looks at them. I have a compliance sheet to show it complies but the plate is wrong....!!!!!! and it has been pranged.