The short answer
Weighing first turns packing into arithmetic instead of guesswork. A CAT scale reads three platforms at once, so you get steer, drive and trailer figures rather than one gross number that hides everything interesting. Take a second pass with the trailer uncoupled and subtract, and you have tongue or pin weight. Then compare each axle against the rating stamped on it. That is the number that decides what moves.
Why does the three platform layout matter to an RV owner?
CAT scales were built for tractor trailers, which is why the deck is split into three pads with visible seams between them. A Class A pulling a toad, or a three quarter ton truck pulling a travel trailer, maps onto that layout cleanly. Front axle on the first pad, rear axle of the tow vehicle or coach on the second, trailer axles as a group on the third.
Tandem trailer axles read as a single combined figure, which is worth remembering later when you are trying to work out why one tire keeps going bald. The scale will not split a tandem for you. It also will not tell you anything about left versus right, because each pad reads the whole width of the vehicle at once.
If you roll out of Eastvale toward I-15 at the Limonite Avenue exit, put the weigh stop in the first hour of the trip, not the last. Numbers you collect on the way home are numbers you will forget to act on.
How do you get a usable ticket on the first pass?
- Load the rig exactly the way it will travel. Chairs, tool bag, firewood, the cast iron, all of it. A ticket taken half packed is a ticket you will have to throw away.
- Set the tanks where they will actually be. Most people leave with fresh water at least partly full and waste tanks empty, so weigh that way and note it on the ticket.
- Put the people in the seats. Two adults and a dog are real weight on the steer axle.
- Roll on slowly and straight, and stop with each axle group fully inside its own pad. Watch the seams in your mirrors. An axle sitting on a seam gives you a number that means nothing.
- Press the call button, give the attendant the number posted on the sign, and stay put until the ticket prints.
- Ask about a same visit reweigh before you pull off the deck. You are going to want one.
Keep the paper. Two tickets taken a season apart, side by side, show how much creep the storage bays have picked up.
What does the uncoupled second pass actually give you?
Tongue weight and pin weight are not printed anywhere on a CAT ticket. You get them by subtraction.
On the first pass, note the steer and drive figures with the trailer still hooked up. Then pull ahead, drop the trailer on level ground off the scale deck, chock it, and drive the tow vehicle back on so the front axle sits on pad one and the rear axle on pad two. The drop in the tow vehicle rear axle number between the two passes is the load the coupler or the pin box was putting on the truck.
The same subtraction works on a fifth wheel, and the number tends to be much larger than owners expect. That downward load lands ahead of the rear axle on a truck bed mount, so it pushes the drive axle up and takes some weight off the steer axle at the same time. If you are working through the numbers on a heavy fifth wheel, watch the steer axle figure fall as the pin load rises. That is steering feel leaving the truck.
How much does a full fresh tank swing the numbers?
Water is 8.34 pounds per gallon. A 60 gallon tank is about 500 pounds, a 100 gallon tank is about 834 pounds, and the water heater adds another 6 to 10 gallons on top. That is not a rounding error. It is the single largest thing you control on weigh day.
Weigh full, dump, and weigh again on the same visit. The difference tells you what the tank is worth and where it lives. If the trailer axle group loses most of the water weight and the tongue barely moves, the tank sits near the axle centerline. If the tow vehicle rear axle drops two hundred pounds when you dump, the tank is well forward and acting as ballast for your tongue weight. Tow dry and fill on arrival and you are running a different balance than your ticket describes.
Why read GAWR per axle instead of just the gross?
The federal certification label lists GVWR for the whole unit and a separate GAWR for each axle. Those axle numbers do not add up to the gross. A rig can sit two hundred pounds under GVWR and still be four hundred pounds over on the rear axle, because the load crept aft while the front stayed light.
Three ratings apply at each corner and the lowest governs: the axle tube and its bearings, the wheel, and the tire at its cold inflation pressure. An ST tire in load range E is rated at 80 psi cold and only carries its rated load at that pressure. Dropping one to 65 psi because the ride felt harsh moves the limit down while the load stays put. Chronic axle overload surfaces later as a sagging frame rail or a spring hanger working loose at the weld, which is when it stops being a packing problem and becomes chassis and running gear work.
What do the numbers actually make you move?
Fore and aft of the axle centerline is a lever, not a scale. Moving a 90 pound tool chest from the front pass through bay to the rear bumper compartment does not just remove 90 pounds from the tongue, it removes considerably more, because that mass is now working on the far side of the pivot and lifting the coupler. The same logic explains why a pair of heavy batteries and a cased portable unit carried as dead weight in the A frame area can put a trailer nose down that otherwise sits fine.
Side bias is the part a CAT ticket cannot see. A galley run, a refrigerator, a slide room and the fresh tank often all live on the same side of the floor plan. Load the pantry and the tool bins on that side too, and one tire per axle carries several hundred pounds more than its partner all season. Individual wheel scales, or a flat lot and a set of portable pads, are the only way to catch it. Eastvale owners who run the same route every month often find the bias has been there since delivery.
What an owner can reasonably do before booking anything
Write the axle ratings from the certification label next to the matching scale figures and mark any that sit within a hundred pounds of the limit. Walk the storage bays with a bathroom scale and write each bin weight on masking tape. If a spring hanger looks tipped, a fender gap has closed on one side, or a frame rail behind the axles has taken a set, bring the ticket in with the unit so the inspection and evaluation starts from real numbers rather than a description. Our other owner reference material covers the running gear checks worth doing before you load again.