Any thoughts on using rv coolant instead of water in the spindle
I use regular antifreeze in mine. It takes care of any possible rust and heat build up. I would also recommend putting a flow indicator in one of your lines.
Unless you are worried about it freezing, water is the better option for cooling. The heat transfer from water is significantly higher and coolant is a mess if it spills. Corrosion is the only other consideration I can think of, and that would depend on the spindle.
Most RV antifreeze I believe are propylene glycol based as it is non toxic and helps prevent freezing. Many industrial spindle coolants also contain this chemical but more importantly have additives to protect against electrochemical corrosion, with some also having anti-microbial/fungal agents. Also, some have pH stabilizers as well. I invested in a Motorex product for my ATC spindle, but it is expensive.
I used anti freeze and then ended up going to the router for a while and it was a pain to get rid of the anti-freeze.