Planning a trip

It's not everyone's cup of tea but when moochdocking, I've been known to water the grass with gray water.

Otherwise, a blue boy and taking it to a sewer clean out in their house works.

And lastly, a macerator pump to a toilet can work.

Or, yes- hit a dumpstation periodically. I like my sewer bladder because I can fill it from all of my tanks at once and drive to a dump station with just it.

Flying J normally has dump stations, $10 IIRC.
Check www.sanidumps.com for local dump sites.
Your gray water should be able to be dumped into a sewer drain. I bought a clear sewer cap with a garden hose fitting on it to do that if needed, about $6-7 on Amazon.
If your relatives have an accessible sewer cleanout access, you may be able to dump black water there.
I bought a maceator pump for my TT. It's like a 12VDC sewage version of a garbage disposal. You place it on the TT sewer outlet and it pumps via a 3/4" garden hose. I've read where some actually run the garden hose into an interior house toilet. As of a couple days ago, on sale for $200 at Camping World.
And I read the relative/friend driveway camping is called mooch docking, vice no hookups boondocking ;)

Search the web for dump stations - they can be found by state. For the closest states, I printed out the results. For large popular states, maybe you could save them in your lap top--or download a couple dozen pages.
Fairgrounds, city, and state parks might have some also.
Good Dumping!!

The answer depends on the size of your tanks and whether you all take showers in the RV. Gray water usually fills up long before black water does.

Your tank capacities would help me answer.

I thinbk you will need to find a place to dump while traveling. Highway rest areas sometimes have dump stations (those along the Interstate in WA do anyway)

Truck stops might, such as Flying J, Pilot, Loves. RV Dealers may have dump stations, or find a campground you can pull in to to use their dump.