How do you know you have solids piled up in the tank? Are you seeing a pyramid like Old-Biscuit mentions or are you just generally concerned?
If just concerned I would say don't worry about it. Just drain the tank, run the flusher for a while to refill it, then drain it again. Add a few gallons of water to the tank to keep things from drying out and put the trailer away. There's just no need to get the tank spotlessly cleaned.
Now if you have a pyramid from too much use with not enough water then you're going to have to get it out. But personally I would just fill the tank with water and let it sit for a week or more and I'll bet that pile dissolves on it's own.
