First off, I don't even HAVE a house, so this is for those of you who still have sticks and brick attachments, or are visiting folks who do. Houses have connections that come in handy for us RVers staying at a residence, and I'm not talking about the electricity and water. I'm talking about sewer connections you can dump your waste water tanks into.
City sewer connections have what's called a clean-out, which is a large opening into the sewer line just as it leaves the house and heads toward the city sewer connection. It's perfect for dumping your gray and black water tank contents if you don't want to drive across town and give people money, which is usually what you have to do when dumping in a populated area. The trick, however, is to find this clean-out, because few homeowners know where they are.
In houses with connections to a municipal sewage treatment system, the sewer line comes out from under the house and usually makes a bee-line for the street where the sewer lines are. If you're in the middle of the block this is easy – corner lots could go either way. Water meters are another utility usually on the side of the lot where the sewer hookup is, so that will help give you a clue to location.
Get a little shovel and gently poke around in the flowerbeds and other stuff along the base of the house's foundation. The line is usually just below ground level, and is a 4 inch PVC pipe (probably cast iron up north where the ground freezes – one of you Yankees help me out here). It's the only huge pipe coming out from underneath the house – supply lines are much smaller. There will be a four inch white plastic screw-in cap with a square protrusion in the middle. Once you have found it, clear enough dirt so none goes down into the pipe when you open it, and get a big wrench or pliers and unscrew the cap. Pull your RV up so that your emptying hose will reach the clean-out, and dump away to your heart's content. Replace the cap and the dirt, remove any evidence of trampled flowers before the homeowner sees them, and you're done.
I have heard of, but never encountered, municipalities where this is illegal – check local ordinances before you do this. There's also a huge difference between city sewers and septic systems. For one thing, with a septic system, the clean-out is going out the back of the house to the septic tank and drainfield, not out the front to the street.
Secondly, septic systems can't handle giant loads of water, so 30 gallons from a Roadtrek or Class B RV should be no problem, but make sure they aren't all taking showers and washing clothes inside when you do it.
Thirdly, make sure you use the clean-out upstream from the septic tank, as close to the house as possible. If you dump into the septic tank itself, do it upstream of the baffle. Dump into the drainfield and you'll probably be invited to stick around for the dig-up-the-drainfield party – as the guest of honor. You don't want this distinction.
When in doubt, drive across town and use the RV dump.