Since my post below received no responses, it appears that this forum could use some info. My trip is now complete and hopefully this report will help out any other folks that happen by looking for info, and perhaps even spark a bit of discussion.
We rented a houseboat for a week in early August and towed along a small fishing boat and a canoe. I won't mention the company because any of the companies will do well, just focus on what you need and see what each offers. For example, the Kettle Falls dam cuts Rainy Lake off from the rest of the park, so pick a company that has access to the part of the park you want.
There was some competition for sites, but we didn't have trouble finding one. Most shifting seems to occur in the morning and note that there are about as many unofficial sites as official sites to choose from. Ask your company to mark a bunch on the maps they give you.
Be open to exploring different types of sites until you find the type that best suits you. Many people seem to flock to the sandy sites due to the nice beach to play on, swim from, and park extra boats on. However we found those buggier and quite annoying because of all the sand getting tracked in everywhere in the boat. Bring a flat pan as a foot bath and remove shoes outside the boat. We actually found that rocky sites often offered better campfire views and weren't that hard to moor to once our group got the hang of it. There are a few "soft" muddy sites as well, but our company warned us about a few that should be avoided because boats get really stuck and need to get pulled out. Ask about those, too.
Our boat had really thin mattresses. We wished we had brought a couple inflatable mattresses to place over or under the ones provided.
Be prepared to fend off a few mice. We had some and judging by the radio traffic we heard, they are fairly common when boats are tied to shore. Apparently the mice have learned to board boats in search of food even if you pull the plank in at night. We put our food in cabinets that seemed sealed and in coolers we weren't using. Consider bringing and keeping your food in bins.
If you haven't been there, visit Kettle Falls Resort area. The hotel/bar/restaurant are worth visiting once, but the dam not at all. The food is kinda pricy, but we enjoyed the served meal toward the end of our trip. The place is NOT very houseboat friendly, so use your smaller boat if within reasonable range. When you visit with a houseboat you see two signs: the first one points to the houseboat mooring area, and the second one on the dock in that area states "no houseboats". You have to find a spot on your own to tie up along a very high rocky shore and take your small boat the last few yards to the dock. Then you have to navigate a very confusing set of trails and paths with no maps or signs until you find the hotel several hundred yards away. It is almost like they don't want houseboats there at all. There is a better docking area for regular boats nearer the resort. Note that the falls themselves are just a very basic, boring dam. Nothing special at all.
Fishing was good once we got the hang of it. We made a point of camping well up into bays and coves so we could fish when others couldn't on the big lakes due to waves. Gold Rapalas are great. We couldn't get any leeches at any bait shop in the area. Go for the smallies and northerns, and you'll find a few walleyes as well. That's what worked for us. Read the fishing laws carefully as several special rules apply to VNP lakes.
The maps they gave us were good, but I'm a GIS/GPS kinda guy. I did some research in advance and learned that the standard map apps on iPhones/droids would be useless in the VNP due to a lack of data connection (they don't reliably cache maps due to copyright issues). So we installed one of the apps below on our phones and LOVED them. We downloaded and cached the maps in those apps in advance and they worked great in real time while motoring around the park with only the standard GPS signals all phones automatically receive. While we could have navigated without them, I loved having the real-time nav in my hand as I motored around all those islands and points that look like all the others. An app company named "Trailbehind" (www.trailbehind.com) has two very similar apps: Gaia GPS and Offline Topo Maps, and either works well. The latter seems to be their cheaper, earlier version but is still great and perhaps even easier to use. Once you have installed either app, be sure to download a set of the aerial photo maps before you go as those are the most current, although the topo maps were helpful, too. These are legal maps.
Be sure to understand and monitor your onboard electrical and LP gas systems. If something goes wrong, the effect is multiplied as the dominoes fall from one thing to the next. Contact your company's base right away if something doesn't seem right.
We really enjoyed fact that our boat had a second helm on the top deck, allowing a very relaxing, scenic view during the hours spent driving around, sitting with the others in our group also relaxing "up top"..
Bring a saw to cut firewood, but don't bother with an axe or other chopping tool. Buy a bundle or two to get you started and you'll find plenty more around to supplement that and keep your fires going.
If you like star gazing, be sure to install one of the apps that tells you all about the sky as you hold it up to the stars. Very nice!
That's enough for now. I will likely think of a few other things to add, but not knowing if someone will even read or need this, that's enough for now ;-)