Look at my sig. My fiver is listed at 39', However it is 40'6" cap to cap. We carry two bikes on the back so when I am like 43' long. We full time and spend the majority of our time in State and Federal Parks. We also use private parks and have traveled in 35 of the lower 48 states. That being said I have always found parks with sites I can fit into. You may have to do a little planning but it is not a big issue. Oh and we full time. We stay away from KOA they are costly and tight. As far as trucks check the specs to see if you can get a bench seat in a 350/3500 otherwise they will only seat 5. You need to know not only the empty weight of the fiver but have a realistic idea of how much weight you plan to carry.
