I lived in Newfoundland (Gander) for 3 years, and went to Goose Bay Labrador for a couple of weeks for work.
1. Bell network only. You may want to confirm that your US phone will bounce off the Bell network as opposed to the Roger's network. They are the 2 main networks, with other companies piggy backing off their towers. There will be many dead spots along your travels. Most people live in towns, with many having cabins back in the woods...you won't see a lot of random houses/farms along your drives.
2. Mid to late June is usually pretty rainy in Newfoundland. Not sure about Labrador. July and August are better choices for timeframe in NL.
3. I'm unsure about RV parks, there are some, but not a huge number. It's not an overly populated part of Canada.
4. Drive during the day...the number one cause of traffic fatalities in NL is vehicle vs moose accidents. They mostly come out at night...and hitting one of them is like hitting a cow on stilts. Beyond that, people in NL pride themselves on being friendly and helpful, so if you have a breakdown, someone will probably stop and help. Just make it clear you are broken down, as people routinely park on the side of the roads as they go in to their cabins, cut wood, go fishing, etc. Crime rate is fairly low, but there is getting to be more property crime as the drug problem started growing.
5. The main road across Newfoundland (the Trans Canada Highway) was pretty good when I was there 5 years ago. A lot of the roads into the smaller communities aren't great. The Labrador Highway through Quebec has a lot of rough spots. I'm not sure how you setup is, but be prepared for gravel roads and lots of potholes. NL is hurting for money these days, so it's not going to get much better anytime soon.
I hope this helps.
2016 Ram Rebel
2018 Coachmen Freedom Express 248RBS
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