The normal rule of thumb for full-timers is to allocate 1500 lbs per person since most full-timers need 1000 to 1500 lbs each.  We are on the low side carrying 2100 lbs last time we were weighed.  But that gives us leeway to have guests with us and to haul something extra from place to place.   Our RV has a CCC of 3900 lbs.
Note: The "CCC" or Cargo Carrying Capacity is the important number and you will have a sticker or sheet somewhere in the RV with that value calculated.  The value is with full water, empty grey and black tanks, full propane, and for a motorhome includes 154lbs per seat belt position.  

How much you can bring along is dictated by only one number as bjoyce said. That is the CCC and that number really can only be accurately determined (as all our rigs are different off the assembly lines) by weighing the combination and doing the math from the rig's (and tow vehicle's) GVWR.

Here's a previous RV-Dreams posting on this...

Certainly, for planning purposes, you can use the published or advertised CCC information. Also, lay out all the stuff you want to take and get out the scale - be prepared to be surprised how fast it all adds up. I weighed everything I wanted to carry before buying and that DICTATED the rigs I could buy.

We have added over 4,000 pounds to our fiver and are just under the GVWR or 16,250. The trailer was delivered at 12,285 and weighed 16,180 when we last weighed it in August, after a year and a half on the road. It's amazing how it all adds up over time. Looking back we were very lucky to choose a model that had such a high CCC. We carry a lot of books and tools which accounts for a lot of the weight, but I doubt that we could get it down to less than 3,000 pounds without feeling deprived. As the others have said, CCC is very important, your not on vacation, this will be your home. Good luck, Fred

We bought our class A, loaded it, then took it to the scales.  Be prepared to rearange, but it turned out, we were good to go!

We added about 3000# to our unit as fulltimers