Well I'm not one to argue with an "islander" like Curval:)
I have never been to Alaska, and never been to Newfoundland so I can't and won't comment on them. I read your post and I immediately thought of two places where I have been in my home province and know a hell of a lot about. I go there every year and it is still exciting and I find new things to see and do.
On the RV idea. Some Scottish relatives of mine flew from Heathrow to Vancouver, BC and did a tour of BC. They loved it and were amazed by the scenery.
Honestly, you can't go wrong sending your folks to the Rockies so BC and Alaska are both good choices. As for finding a place where mom can be comfortable and dad can be adventurous that is hard. Once again I will plug the Jasper - Banff idea.
Your folks can fly into Calgary and rent a car, and drive about 2 hours to Banff. Stay in any number of nice hotels that are close to shopping (for mom) and dad can hike, horseback rides, fish, canoe, kayak, golf, whitewater raft, etc.
After a day or two when dad gets bored they pack the car and head north to Jasper National Park. Trust me their jaws will drop as they round every corner. They can stop and visit the
Columbia Ice Fields http://sca.uwaterloo.ca/~praetzel/BC-trip2/Columbia-Ice.htmlThere are several nice hotels in Jasper and shopping etc. They can ride the tram to the top of Whistler Mountain and see
this (Jasper Prak lodge is on the right where the 3 little lakes are) And there are all kinds of thinngs that would take too long to list here. After they are bored they can drive to Vancouver, which will take about 9-10 hours driving straight through or 3 or more days if they are stopping to see some beautiful country.
I could go on and on, but I won't. All I can say is Canada is huge and driving distances are long. From Banff to Vancouver is probably 8 - 10 hours. You father can't see everything in a week or two without killing your mother.
The other idea I like in this thread is the train tour of Alaska. I would love to do that.