We have sailed for 15 years (1.5 times around the world) with steel Propane tanks without a problem! Key to success is probably the fact that they are very well covered from any spray and it is only the humidity that causes a problem. I did not even put any special rust painting etc as you propose. Another point about steel is that I would not use “Rustoleum” paint or any paint that is not "marine paint". I did try and finally selected Pettit paint rust primer (Rustlok Primer 6980) with best result. There is a very noticeable difference between the primers and the end result (prevention of rust).

Aliza is a Bruce Roberts 53G built from steel so rust is a major issue and you can see some work I am doing now in my web site. (The attache photo is the propane storage area which covers the tanks)