This is a problem of visibility, marketing and lack of know-how, with some politics trow in.
People often only perceive visibility of work being done when things go wrong and they need to be fixed. If you do not go out of your way to show work done - if you often work fix quietly things on the long term and do not inform upper management of the time spent/work done, people may think you are slacking off.
As usual, document what your activities and projects, the time spent, and be sure to err on the excess of emails reporting improvements, projects done and things fixed.
Performance reviews need documented evidence. Be sure to have profusely lists of tasks done, how they improved the current and most importantly, the future state of affairs, and lists of machines/tasks/stuff created ordered by date, together with excel tables with evidence it was all communicated to upper management via emails.
IT people are usually often notoriously bad at passing up the hierarchy what they are doing, preparing baseline documentation sending announcements of what they have been doing or need for the next projects, and then non-technical people think nothing is being done to improve things.
You clearly have two options, going the route of the other guy, or marketing your work and keeping improving things.
I would also monitor systems and provide graphics of the health of systems and weekly statistics of service on paper, to show service done to upper management. They kind of love those beautiful number and graphics in paper. The key word is visibility.
I would also try to get support from some senior element on management that could advise how to play the game on the political/documentation/bureaucratic side. Over time such person could become a valuable ally.
Ultimately, this could also be a cultural problem. Often organizations whose focus is not the IT business do not value, do not know how to value, or even do not want to know how to value IT work.
PS. Have you done an audit/filled up documents and excels of the state of things before you picked up the work of the other guy? Have you documented what you have improved until now? Do not be shy of recommending external contractors to help in fixing something you cannot or do not have time to. Have you already sent a list of equipment and software you will need to do more improvements?
PS2. I have fallen into some of these pitfalls in the past and learn some of them in the hard way. If you want to be somewhat protect yourself and your work politically, you have to learn a bit how to play their game. I also had a couple of mentors over the years, especially an older and savvy general manager that supported me a lot in an hybrid IT tech/managerial role.