I still recommend the power washer clean out, it really does a tremendous job, although using the Unique Tank might be worthwhile trying too if it does a good job. As we are leaving the TT for 6 months we would still use the method below to give it a good clean before storage.
We considered getting someone to come out to clean our tanks when we had a problem last year in Vegas but for less money - just under $200 (after watching how a professional did it on the internet, link below), we decided to buy a power washer and a special jetter hose from Amazon plus a clear elbow which was fitted between the drain and the drain hose, Steve drilled a hole in the clear elbow to get the jetter hose in and up the pipe. It totally cleared out the grey and black tanks. There was a lot of scum in the grey tanks which was slowing down the drainage. If we do this again I will make a video so that others can see it. One thing to consider though is how many bends your pipes have, ours are relatively simple so it worked great for us.
https://www.amazon.com/gp/product/B00LX8Z03K/ref=oh_aui_detailpage_o02_s01?ie=UTF8&psc=1
https://www.amazon.com/gp/product/B01E0DPXFY/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1
https://www.amazon.com/gp/product/B000NCS7GE/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
Here is the YouTube video that Steve watched which made him go with the solution above. You need to ensure the pipes don't have too many bends or the jetter can't get up to tank. We couldn't do kitchen grey but did bathroom and black tank.....it was incredible how much stuff came out.
https://www.youtube.com/watch?v=D1CKK2xIFr4