You've described classic symptoms of a loose or high-resistance neutral connection, and your reasoning is sound. Here's a breakdown of why your assessment is likely correct and where the problem is most likely located

Why a Loose/High-Resistance Neutral is the Culprit
Unbalanced Voltage

A good neutral provides a low-impedance path for current to return to the source. When the neutral connection is poor (loose or corroded), it introduces significant impedance (resistance). This causes voltage imbalances: Circuits with a higher load draw more current. Because of the neutral impedance, this causes a larger voltage drop across the neutral conductor. The voltage to neutral on the heavily loaded phase will be lower because more of the voltage is being dropped across the neutral. The voltage to neutral on the lightly loaded phase will be higher because less voltage is being dropped across the neutral.

Balance Restored with No Load

When all breakers are off, there's minimal current flowing. The voltage drop across the problematic neutral connection becomes negligible, allowing the voltage to each phase to balance out to approximately 120V.

Imbalance Returns When Circuits are Loaded

As soon as you turn on a circuit, current starts flowing. The higher the current, the greater the voltage drop across the high-impedance neutral connection, leading to the unbalanced voltages you observed.

Specific Circuits Causing the Issue

The fact that specific circuits exacerbated the problem strongly suggests those circuits have a higher load (i.e., draw more current). These high-current circuits are making the bad neutral connection even more apparent.

Where is the Problem Most Likely Located?

Given your findings, here's the likely order of locations to investigate, from most to least probable:1. Neutral Connection at the Transformer (or Service Drop)

This is the most likely culprit. A loose or corroded neutral connection before the meter will affect the entire service, producing the symptoms you observed. This is the utility company's responsibility.2.

Neutral Connection at the Meter Socket

This is the second most likely location. A bad neutral connection at the meter socket will affect the entire service downstream of the meter. The utility company may be responsible, or you (or the homeowner) may be responsible, depending on local codes and utility company policy.3.

Neutral Connection at the Main Panel (Subpanel)

Less likely, but possible. A loose neutral connection within the main panel could cause these symptoms. You've checked from the bus bar to the line side of the meter, so you may have eliminated this. However, double-check the neutral connection at the main breaker itself and the connection from the neutral bus bar to the grounding electrode conductor.4.

Faulty Neutral Connections on Branch Circuits (Inside the Mobile Home)

This is the least likely scenario, but not impossible. Multiple bad neutral connections on different branch circuits could create the aggregate effect you observed. However, it's rarer to have multiple independent neutral problems simultaneously, and a single bad neutral typically only affects circuits served by that neutral.

Why you likely have a main neutral issue and not branch circuit neutral issues

The specific pattern of balancing out when completely unloaded and the fact that turning on specific circuits, no matter what their location is, caused a large imbalance tends to indicate a main neutral issue rather than branch circuit neutrals.

Steps to Take1. Document Everything

Keep detailed notes of your voltage readings, which circuits cause the greatest imbalance, and the steps you took to diagnose the problem.2.

Prioritize Safety

Do not attempt to repair any connections before the meter socket yourself. This is extremely dangerous and usually illegal.3.

Contact the Utility Company Immediately

Explain the symptoms clearly and concisely: "I have a significant voltage imbalance on my 240V service. Phase A to neutral is 170V, and Phase B to neutral is 70V. Phase to phase is 240V. When I turn off all breakers, the voltage to neutral balances to 120V on each phase. Certain circuits exacerbate the imbalance when turned on. I suspect a loose neutral connection on the service drop or at the transformer." Emphasize that a loose neutral can damage appliances and create a fire hazard.4.

If the Utility Company Doesn't Find a Problem

Carefully inspect the neutral connection at the meter socket (with the power off, if allowed – and only if you're qualified and comfortable doing so) for corrosion or looseness. Consult a qualified electrician if you are not sure, or if your local code doesn't allow homeowners to perform this maintenance. Thoroughly inspect the neutral connection within the main panel (with the main breaker off). Look for loose screws, corrosion, or damaged conductors. If you still can't find the problem, consult with a qualified electrician.

Important Considerations Appliance Damage

A floating neutral can cause serious damage to appliances and electronics due to over- or under-voltage. Advise the homeowner to turn off sensitive equipment until the problem is resolved.

Mobile Home Wiring

Mobile homes can sometimes have unique wiring configurations. Double-check that the grounding and bonding are correct according to the NEC and local codes.*

Licensed Electrician: If you're not a licensed electrician, strongly recommend that the homeowner consult with one. Working with electrical systems can be dangerous.By systematically checking these areas, you (or a qualified electrician) should be able to pinpoint the source of the loose neutral and resolve the voltage imbalance. Be sure to inform your customer about the seriousness of the situation in the meantime. Flag for review