Call Hopkins (hopkinstowingsolutions.com)+ tell them what you are doing. Bus to trailer wiring can get funky with seprate brake + blinkers going to a single bulb. I had one of their splitters. It worked odd-the parking lights would stay on. I called tech support + told them what I had. "Oh-you want model XXX. Give me your address + we'll send you one." They did-FREE!!!
They make some good multi style all in one units. Heres a kit for an example.
Negative Input Short Proof Power Converter
Another suggestion. etrailer.com. Can't their selection or prices.