On a fair note, there are a LOT of RM developers who put their games out on Steam with little to no polish. I can't comment on your project unless I see it myself, but the main idea of greenlight is presentation. You have to make the audience understand that your game is worthy of being on steam, and that you have put significant effort into making the game. One of the biggest drawbacks of the system is that people will judge based on what they see, and small RTP edits, community resources, tilepacks,, and custom assets mixed with RTP can go a long way into making a good impression there.
My game uses RTP, too, but when I put it on Greenlight, I got around 47% Up votes with not a single harsh comment or criticism. This surprised me a lot because I was prepared in advance for the harsh commenters and trolls to bash on it for using RTP. If you don't believe me, you can check out the link below. Same can be said for many of the other RM games that passed greenlight recently.
https://steamcommunity.com/sharedfiles/filedetails/?id=760430171
One of the easiest ways to bypass the RTP hate if you don't have the funds for custom resources is to use a lot of community resources in your presentation. The RMW community is one of the best I've ever seen, and there are countless contributors who allow you to use their resources commercially without asking for anything in return. While I am yet to thank them personally, words can't describe how grateful I am to them. Use them, they are meant to be used, as long as you can follow their terms. While the Steam community may have grown accustomed to the overused no-edit RTP, they are not hardcore developers who keep track of every resource that gets posted here.
Second, you can do simple edits to your tiles to show that you've done something yourself. Simple rounded caves, walls, recolors, etc. can do a lot for you.
Finally, even if your game consists of 80% RTP, try to present the 20% which contains some degree of customization to the masses as selling point screenshots.
Hope this helps.