APU is something that they'll catch on a closer look. 1/2th tank of fuel is 60-80 gallons, which is a good bit over 400 lbs.

Weights are complex overall.

First, the max weight total is 80k. To haul more, you need an overweight permit for each state you ate passing through, OR something like the FEMA weight permit for emergency responders (which massively increases your weight and every state has to accept it).

Each Axle has a maximum legal weight limit of 20k lbs. However, Drives and Trailer wheels must be below 34k lbs combined.

Steers have a different weight limit depending on manufacturer. It's not 12k - My current teuck is 12.5k, and a previous truck I was in had 14k. Generally, steers are good to slide onto - most weigh stations wont bat an eye as long as you are below 14-15k on em, even if you are above the manufacuters limit. If you get inspected though, they may hit you on that.

Certain additions to trucks and trailers further increase weight limits. APUs add a 450 weight limit increase to Drives and Steers, so you can go to 34.450k, and Reefer trailers adds to the weight limit. to Drives as well. There are other things in addition.

You also then have to take into account Bridge Laws. CA and MI are both sticklers for it, but alot of states have them even if they rarely enforce em.

With Bridge laws, your trailer tandems can only be slid so far back (and in a couple cases, only so far forward) before you get in trouble for it. So you gotta keep these in mind as well.

Gotta be weight legal, gotta be bridge legal.

Enforcement is another matter entirely. Some states/weigh stations dont care if you are over. Some care greatly - Indiana is known for this, and they can and often will penalize 10k over if you aren't legal.