It sounds like you have it covered. From what I have read about parking trailers on dirt/grass, your chief concern should be keeping bugs from climbing/making tunnels up the wheels & tongue jack. The nice thing about putting everything on concrete pads is that it is easier to quickly check for bug problems.
In addition to the pads, one poster (somewhere) stressed the importance of treating the pads' area with bug killer. Moth balls would probably be a nice addition, but I personally tire quickly of the smell.
I would just leave the grass intact, simply because any type of loose ground covering (gravel, mulch, etc.) will probably end up sprouting weeds, and looking nappy after a while. Attempts to string-trim or lawn mow it could sling damaging pieces of ground covering onto you or your project.
Also, if this is to be the Ambassador's home for the shell-off restoration, make sure & leave enough room for "two" trailers if you know what I mean. Lastly, don't forget a pad in front of the steps.