After reading reviews of both the Barker and Thetford totes (42 gal barker, 35 gal. thetford 4 wheel model) it's a wonder either one of those companies are still making them. (Note: all reviews were taken from the AMAZON website)
The Barkers (as sold on Amazon) seem like they were made by Helen Keller and the Keystone Cops. The rear axle in almost every case had some how popped off between leaving the manufacturer and reaching the buyer. In many cases, the handle was not in the box (one buyer said that the seller (not Mfg) was pulling the handles and selling them for $30 - not confirmed). The hose fitting would not fit (as if they were made to a wrong specification). One user said that when he hauled his to the dump, the bottom sagged so much that it wore a hole in the tank, leaving a "snail Trail" behind him.
The Thetford version wasn't much better. The handle is not very sturdy. Broke off on a couple of buyers. The hose o-ring falls off (and into the drain in some instances). The wheels are not as good as the video shows. The filler vent/fill gauge either did not work, or allowed content to vent out the top.
I understand that probably thousands of people have purchased and use these items and have probably never had a lick of trouble. Just do your homework, IF POSSIBLE, go to a CW or other RV supply place and see if you can actually LOOK at a model...touch it, get an idea of what might work or be a problem down the road.
off my soapbox...back to work.