11 MPG is about average for towing heavy loads (around 4k lbs and above) with the RL.My recent tow venture from El Paso TX to Tacoma WA (approx 1800 mi) I in no way set any land speed records, matter of fact, i had cruise set at 70 pretty much the whole way. Was slightly disappointed with it's up hill power considering i wasn't towing THAT much (8200 lb including the truck though oddly shaped). But i averaged about 11 mpg for the trip. Got the trailer stuck in the mud in Utah and blew out a tire just before crossing one of the bridges out here... Hell of a trip lol
It's good you carried a spare tire for your trailer; I know many people with trailers that don't carry a spare. It's hard to tell from the photo, but the trailer tires do look small and lightweight. Do you know the speed rating of your trailer tires? Most light-duty trailer tires are speed rated at 62 to 65 MPH and if you exceed that speed for extended periods they can fail prematurely.
It also looks like there are no trailer brakes. If true, how did your RL perform at stopping your rig on this trip? I'm curries because the RL's stopping performance when loaded is not great and I've personnaly had problems in this area.