Thanks for the additional info man! Worst case, I thought I would go to "thetrack" on Soi 15, but I did not see fitness24seven, it may be a good alternative (at least, it is a cheaper one)!
About eleven hotel: before asking here, I sent them an email saying that I could not see what I was looking for on the pictures from their websites. Their answer was: You can see the gym facilites on our website