Well, this rather quickly turned into a gas vs. diesel debate.
Back to the original question, we have a 37' DP and have never stayed in a commercial RV park. Almost always stay in state parks and have once stayed in a national park (Zion). Most of those sites are electric only with 30A power, which is just fine for us if it is a short stay.
My thoughts are:
National Parks -- I would say that the majority of sites probably don't accomodate anything bigger that 30' and many of them are even smaller. Probably doesn't make too much difference whether you're 37' or 45'. The few that will accommodate something bigger probably would not accommodate a 45' rig.
State Parks -- Most of the state parks we have been interested in have at least some sites large enough for our 37' rig. You do have to choose carefully and fortunately, many parks now have photos of the sites online that allow you to see them before you arrive. For example, when we went to Deadhorse Point SP in Utah, out of about 20 sites, only 3-4 would have been big enough for our rig. I was able to figure out from aerial photos on google maps and other photos of the campgrounds which sites those were and booked in advance. That said, in many state parks, a 37'-40' rig is going to be pushing the outer limits of most of the sites. I can't think of very many that would accommodate a 45' rig.
IMHO, 37' vs. 45' does make a big difference if you are like me and want to primarily stay at state park campgrounds.
Edit: I added a photo of our 37' rig in a typical Illinois state park campsite. Note that there's no pavement (sort of gravel) and not much room at the front of the site for another vehicle. I would not want to take a much heavier, longer tag-axle rig into this site. However, the park is beautiful, and just how we like to camp.