I checked the elite benefits T&Cs (https://www.marriott.com/rewards/terms/elite.mi) and from there it looks like there is no bed/room guarantee for elites at RCs.

With that said, beyond the elite guarantees, if you made a reservation and it states that you are guaranteed a specific room/bed type, then I would do as you did and push the issue. They should honor what they agreed to. But I will say that from your description of the situation background, it sounds like this RC generally treats rooms with 2 beds differently (probably charge more.) And as such someone noticed that and realized that maybe they shouldn't have agreed to this reservation. So they unilaterally changed it (not a nice thing to do after the fact.)

What RC is this stay at? I wouldn't mind checking the various rates they have for some of their different room types. But at the end of the day, at least you did get the room type you reserved and wanted. ^