You will not really be able to determine your total length until it is all hooked together due to several variables.
To get close without it all hooked together:
Measure from the front bumper of your truck to the center of your rear truck axle. (Center of rear hubcap)
Measure from the fifth wheel trailer kingpin to the back of the rear bumper.
Measure from the center of your boat trailer ball hitch to the back of your boat trailer.
Add these three measurements together and then add 5" (for the hitch ball between the fifth wheel and boat trailer).
This will get you within a few inches of total length for legal purposes of determining total length (for Arizona).
My combo is a crew cab GMC + 26' fifth wheel + Ranger Z-19 = 63.5 feet
The federal standard for length and combinations does not apply to recreational vehicles.
Individual States determine maximum lengths for recreational vehicles and permitted combinations.
ARS 28-1095
5. A recreational vehicle as defined in section 28-2153 if used in combination with two units and if the combination does not exceed
sixty-five feet in length.
3. A recreational vehicle may pull two units if all of the following conditions are met:
(a) The middle unit is equipped with a fifth wheel and brakes, and shall have a weight equal to or greater than the rear unit.
(b) If the rear unit has a gross weight of three thousand pounds or more, it is equipped with brakes.
(c) The total combined gross weight of the towed units does not exceed the manufacturer's stated gross vehicle weight of the towing unit.
H. For purposes of this section:
2. "Recreational vehicle" means a motor vehicle that is designed and customarily used for private pleasure, including vehicles commonly called motor homes, pickup trucks with campers and pickup trucks with a fifth wheel trailing device.
A. Notwithstanding the provisions of section 28-1095 that limit the length of vehicles and loads:
2. ...the load on the rear vehicle of a combination of vehicles shall not extend more than six feet beyond the rear of the bed or body of the vehicle.
B. The extensions beyond the front and rear of the vehicle as provided in this section
are not included in determining the length of the vehicle for the purposes of section 28-1095.
Remember, each State has it's own rules and laws which determine maximum length and how it is determined.
http://www.towingworld.com/articles/TowingLaws.htm
http://www.woodalls.com/articledetails.aspx?articleID=1195129