[JRTJH]

OK, let's look at this from a perspective of "will it damage the trailer?" (NO) and "Will it absolutely stink and be unpleasant?" (again, NO, not absolutely).

If the holding tanks don't leak, there's no reason to be concerned with the contents "drying out"....

If the holding tanks start to stink, then it's pretty easy to do what you'd do this afternoon if you were to "go dump the tanks immediately"... in other words, if it becomes a problem, you can deal with it then as easily as you can deal with it now....

What I'd do is add some "TST Orange Tank Treatment". That's what I use because it's what is cheap and seems to work to "keep the stink level" down to a manageable level. I'm sure any other "reasonable tank treatment" would do the same thing, so I'm not advocating for TST, rather for "some type tank treatment"....

About the "worst thing" that could happen by leaving the black tank with a "partial load" (pun intended) is that it would "soften and not be lumpy" when you do flush....

For me, I don't see any problem (from a trailer damage or harm) perspective. And, if it does start to stink, deal with it, if it doesn't, then dump when you next go camping.