Air taxi Captain reported a loss of directional control while attempting to exit runway due to poor braking conditions. Aircraft collided with snowbank and was subsequently towed to the ramp.

Narrative

After landing on Runway XX airplane was unable to make the turn off the runway; and almost went off the end of the runway and into the runway end lights. Upon touchdown which was at or before the 1000 foot mark; I brought the props full forward and put the engines into beta/reverse thrust; applied normal braking and everything was normal. The runway condition was 70% dry pavement with the remaining appearing to be packed snow. As I approached the end of the runway and taxiway turn off. I applied normal braking and differential power to start making the corner. The airplane would not turn and I could feel the brakes were slipping. I increases the differential power and working the brakes and nose wheel steering; there was little change the airplane was sliding toward the end of the runway and toward the runway end lights. Momentarily applied reverse trust to both engines; then shut both engines down before hitting snowbank or runway lights. The airplane stopped before hitting anything. The airport manager came out with a tug and was able to hook on and push the airplane back and towed me to the ramp. There were no NOTAMs regarding the runway condition. After walking back out to the runway I would say the last 500 ft. of runway were braking action 1 (Poor). The rest of the runway was a 3 (medium). The airport manager drove out to the runway with the sand truck before I could get back out to evaluate the runway for myself. The airport manager told me he had sanded the runway the night before. There was no sand remaining when I landed. I suggested he apply still more sand as it appeared to me that 50% of the ice was still bare (no Sand) on the west 500 ft. of the runway. The manager agreed to do so and informed me that the sander on the truck was jammed and he would do it be hand with a bucket. When consulting the airport manager on runway conditions before departing to ZZZ I have found the occurrence of field conditions to be questionable as to the accuracy; and like this morning there was no NOTAM which would imply good runway conditions; when in fact the last 500 ft. were poor braking action at best.Suggestions: Better snow and ice removal; Better NOTAM issuance.