I've done the same as both of the above -- we only camp with electric, no water or sewer. Replaced the Thetford with a Curve, which means unlimited black tank -- it's a simple matter to dump the cassette into any pit toilet available. The grey water tank I've not filled in 4 days, but we also tend to reduce the use of that tank by washing the majority of the dishes in an outside sink that drains into the bushes via hose. I have no issues with this, as it's not any different than if we were tent camping, used a wash basin, and dumped it into the bushes.
As to the grey tank itself, you can get a replacement cap that has a screw-on port for a hose, like this:
http://amzn.com/B0006MRR2Q/ If you ever find your grey water full, you can drain it judiciously into either a bucket, a carrier, or if it's something allowed where you're camping, you can water the plants.