Portable Sewer Tanks

Looking for any comments on the portable sewer tanks that you can pull with a truck. We do a lot of camping in sites that only offer water and electricity and we get close to filling the gray and black tank sometimes before leaving (usually only camping for a day or two). It would be nice to not have to worry about hitting full on the tanks and then having to break camp to go to the dump station. I have been looking into these portable tanks, but don't know to much about them. Where do you put them when traveling? How clean are they? Any help would be appreciated.

I have a 40 gallon one with 4 wheels. I have used it a couple times. I carry it, bungeed down, in the bed of the truck, behind the fifth wheel. They work fine as long as you remember you have to drive slowly when hauling it (full) to the dump station. The one I had previous to this one had a design flaw (I believe) and it cracked around the back axle. We take ours with us if we are going to be out longer than a weekend at a park with no sewer connections.
The only problem is sometimes the dump station connection is higher than the side valve on the portable tank and you find you have to "help" it drain completely by lifting the frontend a little...40 gallons of water is HEAVY!

We have one that we use when we camp without full hookup. I have never used it for the black water, only the gray.If you only use it for gray water it will not be to messy.I usually just rinse it out when finished.

I modify the sewer hose to a shorter length. I have a 15 gallon tank very handy size. Its a slower drive to the dumping station than its bigger brothers,but easier for me to handle and smaller to store. Plus I figure that I got the time to make a second trip if need be!!