From a post here.
There are 2 main manufacturers of the tanks: Barker and Thetford and each manufacturer makes different sizes and the price goes up with each size bigger.
There are 2 wheeled versions and 4 wheeled versions.
Of course the 2 wheeled versions means you have to lift one end up to move it around. The 4 wheeled versions means you simply have to pull it to move it (no lifting).
Tretford uses the same hose to fill the tank as it does to dump the tank. Barker has a top fill and a slide valve side dump.
I tried using the Tretford and absolutely hated it because of the single hose. When dumping, all I ended up doing was making a poop mess everywhere. It was my in-laws tank.
I’ve owned a Barker 32 gallon 4 wheel side discharge valve version for over 20 years and it’s still going strong.
Here are a couple tips:
The older you get, the heavier they get. So, get something you don’t have to lift or carry.
Get something that will adequately hold one of your tanks without overspilling. Your holding tanks are 30 gallon, then get at a bare minimum a 32 gallon tank. You will never over fill it that way and make a mess at the campground (or your own yard or driveway).
You could get the 42 gallon tank, but with your tanks being 30 gallon each, if you were to dump 2 tanks into a 42 gallon, you run the risk of overspilling 18 gallons on the ground. The extra 18 gallons will not benefit anything. Make 2 runs to the dump station if you have to. You’ll have to so anyway, even if you have a 42 gallon tank. So get the tank that will comfortably hold your largest holding tank and you will never over fill.
Weight? There are those who say the 4 wheeled 32-42 gallon tanks are just way to heavy. Yes, they are. But you are not lifting them in the air, you are rolling them on the ground. Big difference.
I’m 63 years old and can still pull my full 32 gallon tank through the grass by hand. I do it at home all the time using my septic tank at home to dump in. I use to tow it behind my small John Deere, but realized it was much easier just pulling it by hand.
The side slide discharge is nice. You simply attach a hose and pull the lever. No spilling anything on the ground. Same for filling. You unscrew the cap on the top, attach your hose and pull the lever on your trailer. No spills, no mess. Although, you will need to lift the hose to finish draining it out.
Barker guarantees their totes for life! After almost 20 years, they sent me a new slide valve replacement absolutely free last year when mine started leaking. I replaced it and it’s working absolutely great again.
I’ve towed mine behind my truck through campgrounds over some 20 years at lest a thousand miles or more. Still works great!