The holding tank of an RV is not the same as a septic tank for a house; there will be little to no breakdown over time. You are right, though, that it's best to let the black tank get as full as possible before dumping it, just to flush everything out easier. No reason not to do the same with the gray tank either. We can generally go 4 days camping without having to dump, though if we are moving on after 2-3 nights, we'll usually dump all waste tanks when leaving the CG, just so we're not carrying the extra weight of the wastewater around.
JMHO, but the cryptic title of your post may not serve to get you as many responses as you might like. I'd suggest titling your posts in a way that we know what's likely to be addressed in them.