I have a Barker, 4 wheel. I really like the capability it gives me, because some of our camp outs dont have dump at each site, and this is easier than breaking camp to empty tanks. I only use it for grey water because I store it in the garage and dont want to stink it up. 3 things to consider: 1. The handle to tow it could be longer. I am tall, and would like it to be longer so I can more comfortably pull it by hand. 2. It can "rumble" when pulling it back, empty, from the dump, I think it would tow easier and quieter if I would get around to putting the lager softer tires on it. 3. The dump valve to empty the tank is on tne side, and a little bit of the tank is delow the valve so getting it completely empty can be difficult.
I bought a "gauge", which is a float that screws onto the tank vent, to get an idea of when the tankmis full. It doesnt work. If it is screwed in it blocks the vent and prevents the tank from filliing. If I loosen it so it vents the floating piece sticks. Dont bother with one of these.
I would recommend a 4 wheel tank with a dump valve on the end to ease tipping it up to get it all emptied out.