While you may be able to drive up to a 45ft RV on the interstates, actually doing the things you indicate are much different:
RV Camping in YosemiteBoondocking will be very difficult in California with a 45 ft RV. First, you are likely not legally allowed down the roads to get to boondocking sites. Second, even if you can legally go down the road, you may not be able to find a spot off the side of the road that will accommodate a 40+ rig. Third, you will have an almost impossible time trying to turn around if you go down a wrong road.
National Forest Service roads will be difficult to traverse with a 40+ foot RV. NFS campgrounds will be pretty much impossible to access. Depending on what exactly you mean by "local lakes", you may not be legally allowed to travel the local roads to the local lakes. Majority of California State Parks will not accommodate rigs that large.
I'd seriously look at why you believe you need a rig over 40 feet and consider compromising at least a little. A 37 foot rig will get you a lot more access than anything over 40 feet - is that 3+ feet REALLY needed?
