It does sound as if treatment will be on-going until his environment dries up.
One thing I would do before adding the treatments others have mentioned would be to get a stiff bristled fingernail brush and brush his frogs out with a warm water mix of povidine (iodine).
I buy WalMart's brand of povidine. You could also add a bit of vinegar or a bit of clorox to the mix. Not a lot of clorox, just a splash.
Pat the frogs dry with a paper towel, then put whatever topicals you decide down into the crevices of the frog. Be sure to include the sulci which is the crack where the top of the frog meets the heel.
If he gets a fungal infection down in there, you will have a dickens of a time trying to get that healed up --- especially when he's out 24/7 and doesn't have many options for dry places to stand in.
So no, you aren't missing anything in terms of treatments, it's the wet/muddy/manure environment he's in. You may find yourself switching off treatments too.
All of the suggested treatments are great ones, it's just going to be a lot of extra time and work for you, I'm afraid
Under the conditions you describe, if it's possible to treat him every other day I would do that. If not, just try to get out to the barn as often as you can thru the week