I am not sure where you are getting your information about campsites limited to a combined length of 30 feet, but my experience is much different.
I am sure that there are probably some Forest Service campgrounds where that might be the case, but almost every private and state parks campgrounds can accommodate larger rigs. I have a 1 ton crew cab that is about 25 feet ling, and I pull a 35 foot long 5th wheel. My combined length is about 53 feet.
In most cases you will likely unhitch from what ever trailer you end up with. You will only need to be concerned with the length of your trailer, not the combined length.
Without knowing how and where you intend to use your trailer, it is difficult to give you a specific answer.