Shimano switched exclusively to freehubs when they went to 7-speed back in 1989 (or right around there). RSX is definitely a freehub. (Shimano has started making 7-speed freewheels in the last few years, but it's a new development and only on rather cheap bikes.)
If you're just using the extra wheel for use on the trainer, it doesn't matter whether it has a freehub or freewheel. And by spacers, I'm talking about spacers on the axle (that go between the cone and the locknut). This applies to both freewheel hubs and freehubs, but if the gears are further than they need to be from the frame's dropout, remove a spacer or two from the drive-side axle and stick them on the non-drive-side.
If you get both wheels adjusted such that they have the exact same distance between the smallest cog and the drive-side locknut, then you won't need to adjust your shifting when you switch wheels.