RVs were made for rolling down the open road, stopping for a bit, then moving along. But sometimes we get "hung up" for a while. Maybe a job requires we stay put in the campground for a couple of months. Having a "stationary RV" means special care of the black water holding tank.
Regardless of whether you're stationary or moving every day, NEVER leave your black water holding tank dump valves OPEN. Even if your rig is hooked up to a sewer line, getting all those nasty solids out of your holding tank requires the movement of a large amount of fluid. More than one RVer has learned the hard way that leaving the black water dump valve open translates quickly into a holding tank that won't dump.
When an RV rolls down the road, it's not just beneficial for us behind the windshield, seeing those new sights. Down under in the holding tank, that rock and roll motion the rig keeps the black water tank stirred up. This contributes to the breakdown of solids and the removal of foul odors. Park your rig and don't move it for a few days, a different setup occurs in the holding tank. Solids tend to gravitate to the bottom of the tank and an ugly sort of transformation begins to occur. We'll spare you the grosser details, but it needs to be said that a goop-like substance can begin to coat the bottom of your holding tank.
That goopy coating is hard to get out of the tank and if not evicted can begin to build up and up and can eventually cause a blockage. To keep this from happening, when you dump your blackwater tank (which should be done ONLY when the tank is at least three-quarters full), allow plenty of time for the contents to evacuate.
Here's where having a clear plastic sewer hose-to-dump port adapter is a good thing to have. When you dump the black tank you can closely observe just what's coming out of the tank. After sitting for a number of days you may be surprised just how s-l-o-w-l-y those contents can be about coming out of the tank. Slamming the holding tank valve shut before the contents are clear can not only leave you holding more gunk in the tank, the stuff can also prevent a water-tight seal on your black water valve--not a good thing.
If you find the tank just doesn't seem to want to "get empty" then leave the valve open and have a helper run the toilet until the tank runs clear. We've found it may require a greater volume of water than the toilet itself will readily provide. We have a secret weapon: Our shower hose will stretch far enough to blast water down the toilet making the job much faster.
In hot weather stationary RVers face another problem: Sour tanks. The combination of heat and lack of motion can cause the breakdown of wastes in the tank to stop, causing a really, really stinky situation. Assuming that you are either not using any sort of holding tank treatment at all, or are using one that's based on enzymes and/or bacteria, you may be able to clear up the problem by dumping a half-cup of baking soda down the toilet with a couple of gallons of water. This may "sweeten" an otherwise acidic condition, getting natural bacteria to begin waste breakdown, reducing the stink factor.
In really tough situations you may need to fill up the holding tank with water, dump it completely, and then add a couple of cups of baking soda and fill it up with water again. This time don't dump the tank immediately; let the tank sit overnight, then dump it and "restart" the system with the addition of a bacterial and/or enzyme based holding tank treatment. We had this happen once during a hot spring stay on the Arizona desert. The stench was overpowering, but the "fill, dump, refill with soda water, dump again" procedure proved a life (and nose) saver.