We just got back from a trip in the eastern sierras of California and at most of the campgrounds a truck and 35' trailer would not fit at all and at other places there were 10-15% of the campsite where it might have been feasible. Most campsites in the west are back in type and not pull through and most are not wide enough to easily put a truck parked to one side - some yes but with many the answer is no.
This is most true for the USFS and BLM campgrounds. And if they have 30 campsites and only 5 are going to work for someone with a 35' 5th wheel what are the odds that one of these will be empty when you arrive? Even for the less than 50% where you can make a reservation you will have very little or no idea which site to reserve for your rig.
I would strongly recommend getting the shortest trailer that you can be comfortable staying in and will provide the space you really need for your gear. The newer trailers with one or two slideouts make more sense than a longer trailer to get more living space while at a campsite.
The federal campgrounds we stayed at cost $10-12 a night whereas the few private RV parks in the area charged $39-55 a night. With the federal campgrounds we were out in the forests or desert or beside a creek instead of sitting alongside the highway listening to traffic or trains.
I would also recommend giving serious thought as to whether to get a 5th wheel or a travel trailer. With a travel trailer you keep the use of the bed of the pickup and it can be used with or without a cap and with racks for kayaks and bikes, etc. for more options when traveling.
