Simplifying code, getting invalid conversion from 'const char*' to 'char*'
The function declaration 

void publishTankValveStatus(bool demandStatus, char topicToPublishTo)


declares topicToPublishTo as a char, that is, as a single character instead of as an array of characters that is constant.

Change char to const char * to say that  topicToPublishTo is an array of characters that is constant.