Jan the holding tank on the Scamps is so small it really is not hard to clean and not really worth spending a lot of money or using up space on extras for it.
A quick and simple way is to just before dumping fill it right up with what ever water you have and dump it, then dump your grey water tank to clean out the sewer hose. A flexible tank wand
is another handy item to have when you have a hose to connect it to while dumping and it takes up very little space - you can put it in from the top or the bottom. But if you are dumping your tank out often you probable will not even need that to often.
Even simpler way to clean a dirty tank is after dumping it to put a bag of ice in it and drive with it.