I've researched dump trailer charging quite a bit over the past few months. I just got a new Moritz dump trailer, and I got drawn into a lot of conversations regarding problems with hydraulics, charging batteries, trailer harness issues, etc. So now I feel like I'm educated enough in all this to keep mine in shape with some preventative maintenance. Some brands offer an onboard charger with the trailers, but Moritz doesn't. Deep cycle batteries need more amps to charge properly and extend their life, so most 1.5 amp maintenance chargers are not the best for them. I plan to buy a 4 amp Noco Genm1 waterproof charger to mount in the trailer tool box, and add a through hull 110v outlet so all that's needed is an extension cord. I'll plug in the cord for at least a day after each day of use.
I've noticed that so far my battery does get charged nearly full by my truck's trailer harness after using it. I think this is what sets up most users for eventual failure. These batteries charge easily when new, and with minimal draw down first. Then after the battery gets some use and age it doesn't get a full charge before parking. That partial charge begins the sulfating of the battery and a quick spiral to lower charging, shorter cycles, and shorter life. Eventually we find a low battery with low capacity.
Anyway, that's what I've found in my research, and it matches what I've found in my own battery use with other equipment. I use a Battery Minder on all my other batteries with excellent results. I needed a reasonably priced waterproof onboard model for my trailer, and the Noco Genm1 seems to be a good option.
