I agree with Pete, and if you're not level, the readings will be off too! Sounds mighty small for a black tank. We carry 100gal fresh, 40 each black and gray on a 33 footer. Never read full on the black yet, (came close Thanksgiving; had guests too) fresh said empty at the end of T-day weekend, but kept pumpin' water anyways. Our gray does fill up, especially on extended weekends....when the bathtub won't drain, that's a sure sign!!
In answer to your question, yes, you can buy 'auxillary' holding tanks. We have been considering adding additional gray storage to ours. But in the meantime, I conserve by using a wash tub in the kitchen sink, dump it in a hole outside or into the black tank. I try to wash as few dishes as possible, but there's always some to do, especially cookware. We also utilize the outside shower nozzle for the really messy stuff. Oh, and I keep plenty of sanitizing hand wipes on board for the anal son...much less running of water that way!!