Fresh water I always carry about 20/54 gallons just to be safe. I fill from home 54/54 if going to boondocking site or where the well water at a campground is poor quality.
Black tank after emptying I fill 5/40 just to have some water in there plus bio pack.
If I only have 1/3 full and need to dump, you can always let the toilet run for a bit and fill some more. When dumping I always leave the black open till empty then dump two 5 gallon buckets of water down toilet to rinse before closing and dumping grey.
I now dump black every time before driving many miles.....last year tank was full, had 6 mile trip down rough dirt road to dump station. Toilet down pipe to tank--plastic spin weld was bad .....enclosed underbelly. Horrible horrible smell.