How much does 30+ feet actually effect ability to find space

It really depends on where you want to camp, RV parks are no problems however if you want to camp in a NP like Glacier or Yellowstone it is really a numbers game. There are some sites that will accommodate 40 ft MHs most will not, so if you plan your trips well and stop early enough you may find availability. The advantage of a shorter MH is that you will find more sites available to you. Most NFS CG's usually have larger sites and are less crowded than the NP's and can usually accommodate larger MH's.