my 31L has very similar sized tanks
Using it and getting a feel is probably the best solution
My only thought is if you can at home or at a site with full hook ups play around with filling the tanks using a 5 gallon bucket of water, or water hose and get a feel for indications for when they are getting full.
In mine there is a distinctive gurgle when flushing the toilet as the tanks are getting nearly full. I think the vent stack gets covered by the high water mark so the toilet becomes the vent. Once that starts I know there are still a few flushes left so no need to panic but better plan on dumping real soon.
Same thing for the kitchen sink, although less pronounced. The water will back up into the shower first, so pay attention to that when you think you're getting full.
Once you understand that, it's a little less stressful to start extending in the real world.
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