I had assumed this was a new trailer, but it must be used otherwise there wouldn't have been enough time for the anode rod to get used up completely. Never thought of that when I first posted, doh!
I bought a water heater tank flusher, a simple spray wand that connects to your garden hose so you can spray the sides of your tank. I was surprised how much sediment I got out of my tank when I first used it - and that was after just 1 year of use. You might want to invest in one of these $6 tools too.
Water Heater Tank Rinser - Camco 11691 - Water Heaters - Camping World