We're looking for a bumper-pull dump trailer to haul the manure from ten horses to the dump, preferably monthly but possibly every two weeks. We're looking at a 7 x 10 or 7 x 12 size with a single-piston lifter. Are we on the right track? Do you have any recommendations? All advice is appreciated.

 

don't know where you are located, if CA unless you have a class A license do not buy any utility trailer over 10k.
the other side of this is that manure is heavy if fresh, make sure you buy a trailer that can handle the weight.
i have a Load Trail 8 x 14 14K dump trailer. if i load it with dry dirt level with the sides rated at 6.22 cu yds; it is 1,500 lb over weight. the trailer weighs about 3800lbs.
don't know if you can weigh a known quantity of manure, say a cubic foot and calculate out your needs from there.
by the way the Load Trail trailer has been back and forth loaded between CA and NM several times plus hauling my track loader or materials numerous times. i've been pleased with it's performance.

 

You don't detail much about how much you will spend, or if you're only looking for a factory set up.

If you want something that will haul a heavy load you might look up one of the military trailers. They're about 6'x8' with 18"-20" sides, all metal. A guy locally has them for $350-$400. New tires, some have duals. Has the side racks, even the tarp. They look like new. They have about a 20" tire so they set high which would be perfect for a dump. Bumper pull.

You'd have to modify the hitch, probably, 'cause it's a pintle, and add the dump cyl, and hinges. It has a rectangular bed so no fenders to stop the load from sliding. It would haul all you could stack on it.

You could build what you need pretty inexpensively and never wear it out and easy to pull.