Let's start back with your tow vehicle. The published "maximum towing capacity" is a rather useless number. In most cases, you'll run out of payload before you hit the maximum towing capacity of your tow vehicle.
On the driver's side door or door frame, there should be a sticker with the vehicle's weight ratings. There should be three numbers: GVWR (gross vehicle weight rating), GVWRF (gross weight rating for the front axle), and GVWRR (gross weight rating for the rear axle). Next, you need to load up your kids, fill the gas tank, get the dog(s) (if you have any), and throw anything else you will have with you camping in the back or the 4-Runner and head for the nearest CAT scales (local truck stop). Pull onto the front two pads (front wheels on front pad and back wheels on middle or second pad). Jump back in the vehicle before they press the button to weigh. You will get three weights out of this: the total vehicle weight and the weight on the front and rear axles.
Next, add the actual weight of the rear axle to the weight of the load-distributing hitch system you're going to use and subtract the sum from the GVWRR on the door sticker. This will be your leftover "payload" for the tongue weight of the trailer.
How do you estimate the tongue weight of the trailer? DO NOT use the dry tongue weight or anything the dealer tells you. Find the GVWR for the trailer (RVs will have a sticker, too - usually on the driver's/street side at the front) and take 12% of that for a tow-behind trailer to use for an estimated tongue weight. If this number exceeds the usable (leftover) payload from the exercise with the 4-Runner, it's too much trailer for your tow vehicle. Stay within this safety envelope... THEN find an RV that suits your needs and wants.
Rob
