We did the same trip last summer. Great scenery, lots of wild life, great people. Almost the whole highway is paved now. There was about 100 miles unpaved between HVGB and Port Hope Simpson. This is also the longest stretch without gas. There is also about 100 miles unpaved in Quebec. They continue to pave and the whole TLH will be paved in 2020. Money just got approved.
Not so sure about campground with electricity. There is at least one in Labrador City and several at the East Coast (south of Port Hope). HVGB has electricity hook-ups at the Town Hall and the tourist information where you can stay over night. Churchill Falls has electricity at the Town Hall building, again with permission to stay over night. We boondocked most of the time.
Make reservations for the ferry. It can get very busy.
If you have time, consider taking a cruise on the Labrador Ferry from HVGB to Nain and back. It takes 5 days return and you will get to see Inuit villages only accessible by boat (and snowmobile in winter). The ferry Kamutik W leaves every Monday and returns Friday. We saw also icebergs and whales. There are comfortable staterooms on the boat.
Enjoy your trip.