Woodall's is probably best for RV specific issues
RVing in Mexico and South America
I doubt anyone coming into Mexico with an RV has to declare what's inside and you come in with a tourist visa like everyone else. When you switch to an FM3 it's just like you drove down in a car. The difficulty might be using an RV park as an address because you usually need to show a bill with your address on it. There may be ways around that.
Yes, when you move to another area that has a different Immigration office you need to do a bunch more paperwork - but you don't start over.
You need to check with Immigration when you leave the country and when you re-enter with an FM3. They will stamp your booklet and do some paperwork.