Black and grey tank not emptying completely

It might be how you are managing the tanks in general. I got these tips from a full timing book. Never let the tanks go dry. You always want at least an inch or two of water and chemical in them, that way when your going down the road the back and forth movement creates a washing machine action and helps the chemical along. Try to have the black tank as full as possible before you dump it. It gives greater gravity action to help expel the waste. Lastly, I use only Thetford Aquachem with the blue cap. I've never found anything better and I been disappointed with the different powders and such. I have never had a problem in 20 years, just keep that tank wet.The fluid level should always be above the waste. If it's been a week or two add another bottle can help. It could be that you have sludge build up near the front of the tank.