Neither of us are a fan of crowds and would rather go places where we can ski powder all day instead of fighting for it with crowds.
That describes Powder pretty well.
The terrain seems to look pretty inviting on Google Earth. Is it decent?
Depends what you mean by "decent", of course. It's not the steepest or longest vertical, but it has tons of fun nooks and crannies to explore - groomers, trees, rock jumps/small cliffs, gullies, marshmallows, etc. Between the day snowcat, shuttle, lifts and off-the-beaten-path short hikes, you can explore for days (seasons really). If I remember right, you like some of the smaller, less-impressive-on-paper resorts in CO, so it should be right up your alley.
And if you're looking for something steeper - or just a change of pace, Snowbasin's right across the valley. Surprised you didn't mention that as it fits your description at the top perfectly, despite the beautiful lodges and pristine shitters. If you've never been, it would be silly not to give it a try if you're that close - both great resorts.
Are there any RV parks or other campgrounds in the area open in Winter?
I'm not an RV camper, but as far as I know there's nothing in the direct vicinity. Powder is up a steep, narrow road that I wouldn't want to lug a trailer or RV up and it's pretty much the only thing up there, save for a few condos around the base. I doubt they allow overnight parking, although there's a pretty prevalent "nobody gives a $h@t, have a good time" vibe (they don't even check lift tickets most of time). So if you parked in a far corner of the lot, I'm not sure anybody'd bother you ... not sure they wouldn't either, and you were looking for hook-ups. ..
A quick Google search came up with a couple of places in Ogden, Layton and surrounding area that appear to be open year round. That'll put you about ~30-40 minutes out of Powder. If it were me, I'd skip those and camp at Crystal Hot Springs a little farther north outside Brigham City. Google puts it at an hour from Powder Mountain, which sounds about right. Farther drive, but you get to come home to a big hot springs pool and jetted tubs, plus a cold spring pool. It looks like they have full hook-ups and appear to be open in winter, but of course, I'd call to make sure.
Is a mult-day ticket the best deal or are there other ways we should try to get discount tickets?
Pretty sure you're not gonna beat the $55/day multi-day price. They sell discount day tickets at local ski shops and the grocery store near Powder (I think), but I don't think they'll be lower.
Is there any reason not to plan a trip like this around late March/Spring Break?
Only weather. March is usually good, but if it warms up, Powder can take a sun beating. Overall, I'd have no problem recommending March, but if you can, "wait and see" is always best.