Your best bet is to get the largest RV you can afford that fits your needs now and at least 5 years into the future. Like others have replied you will be able to find a site in or near most tourist/sight seeing destinations.
We travel in a 38ft 5th wheel and have always been able to find a place to stay wherever we have visited.
When deciding on the size I always use this measurement...if we are stuck inside the RV for several days due to inclement weather would I be more comfortable in a 25ft or 38ft RV. 364 square feet vs. 250 square feet provides much more room to get away from others when cabin fever begins to kick in.