50 AMP SERVICE ?

We basically use Trailer Life and AAA Camping guides. I think those two are the best printed sources.

For on line I use TL and KOA. We aren't KOA members but three years ago we did a mid-September to Thanksgiving road trip and found very few parks open and relied on KOA. A KOA membership would have paid for it's self that trip. I used RVparkreview.com the other day and it lacks detail but does have some good info. DW will Google campgrounds on occasion and loos up websites

I have gleaned a lot of good info from reviewing posts here before we leave and DW checks them out in the printed lists.

We seldom reserve in advance and we don't have smart phones and don't always have WiFi on the road so the books are best for us.

I think I'd just go with http://rvparkreviews.com/ to find the CGs in the areas you want to visit, and then check their websites.
I would rather see the country, and the sights, rather than plan my trips around 50A hook-ups.
One of the nice things about RVParkReviews, is that it give you listings of the parks that are also in the area of a certain town/city, so if you find that one place doesn't have what you want, you can search without having to pull out a map.
Also, since the CGs are rated by folks like you and me, you can get a better sense of what is good/bad about them.

I also think that a majority of the CGs out there now, have the 50A, even if only for a few sites. Availability may be more of an issue than whether or not a CG has it.

That's a tough question to answer, given that campgrounds change from day to day almost. For sure, the best list is whatever one you got and a phone call ahead to reserve a spot with what you want in the way of amenities.

I use 3 different smartphone apps, and while I don't care about 50 amp service, for everything else, I've found that none of them covers everything as accurately or thoroughly as I'd like, but with the 3, I usually can find one good place to land.

Two of the three were free, the third cost a few bucks, around 10, I think. If you have a smart phone, the three I use are Good Sams, Woodalls, and Allstays. The last of these has the largest list, but is short on showing fees. It often shows campgrounds neither of the other two list, however.