May as well find out what it can do by setting it on 7. It won't speed up col-down time much, tho'. Best to install a remote-reading thermometer in it, so opening it to check won't slow down cooling. Empty, a fairly good operating RV reefer can need 12-24 hours to fully cool down. Fill it with pre-cooled stuff from the house reefer, and only add small amounts of warm stuff over time.
It (the reefer and RV) needs to be darned close to level. You can ruin the cooling system by running it off-level much or for an hour or so.
The noise may or may not indicate issues. My Tiger's much-abused reefer clucks quietly sometimes running on propane, but not noticeably on electric. It cools OK, but not great.
Condition of the door gasket can be an issue with cooling, and Dometic may or may not have one (probably won't, in my experience).
Jim, "Manure occureth."