45' motorhomes are apparently not permitted in DC, Rhode Island, Maryland, and New Jersey, as well as the majority of Canada. How vigorously the laws are enforced (particularly in the states) is quite possibly another matter. States also have various limits for combined length when towing, and a 45' motorhome plus a toad would run over many of these limits. (Again, enforcement of the laws may be quite weak in many cases.)
While there's not really a sudden cutoff at 40' (so far as I know), there are many older campgrounds in the East with limited availability for larger and longer RVs, and any increase in length in general will further limit the sites available. In the Vermont state parks, for instance, many of which were constructed by the Civilian Conservation Corps, there are some sites that are tent only (walk-in with no vehicle access), some that are only large enough for a smallish trailer or perhaps a class B, and so on, with only a few that would be comfortable for even a 40' motorhome. I have a somewhat limited selection at times with a 32' motorhome with no slides. If you only fit into one or two of the sites at the campground, and those happen to be in use (even if many others are free), you're obviously out of luck. I guess I'm just saying that every configuration and size has its advantages and disadvantages, and there are trade-offs no matter what you end up doing.