Definitely put stuff away in cabinets or in the sink. Even if you have things that tend to stay put on the counter pretty well, in the even of a sudden stop or a collision they can very readily become airborne missiles. (If you have a trailer, they won't tend to actually hit you, but still can wreck havoc on themselves and other things in the RV; and often trailers tend to bounce around more than motorhomes.)
Comfort level kind of depends on the person and on the conditions. It doesn't take too long to get rather more comfortable than when first starting out, but things like construction zones or nightime driving (or nighttime driving through construction zones, should you be so unlucky) take somewhat longer. Careful practice is about the best antidote.