RidEx makes an RV solution similar to their septic tank enzime that works well to digest the solids, also a pound or so of bakers yeast and a pound of sugar gets things going to, but it takes a while.
I keep a small barn heater under my 5ver on low, have the unit skirted, and have dumped my tanks at +5 air temp with no problem. I dump both my tanks when the grey water gets full, and use a lot of water into the black water tank, I also have a heat cable running from inside the sewer drop up under the dump line to be sure it doesn't freeze up.
Just some ideas when it gets real cold and at 30 it hasn't even got cold enough to freeze up the water lines much, like stated above, I think you have a solids build up and may need to get a hotsy or pressure washer to brake it up.
