6.5 RV Onan generaotr wont kick in when loaded. checked automatic transfer switch and excited it manually, worked one time but never again,

Here is some reasons for reference:
1.After the diesel generator is stopped, the residual magnetism is often lost, because the material used for the magnetic pole of the exciter is close to the soft steel, and the residual magnetism is small. After the shutdown, when there is no current in the excitation winding, the magnetic field will disappear.
2.The magnetic pole of the diesel generator loses its magnetic properties.
3.Damage of excitation circuit components or circuit breakage, short circuit or grounding phenomena.
4.The exciter brush is not in good contact with the commutator and pressure of brush holder is insufficient.
5.Excitation winding wiring error, opposite polarity.
6.Stator winding and rotor winding of generator set open circuit.
7.Poor contact between diesel generator brush and slip ring, or insufficient brush pressure.
8.Diesel generator lead wire wiring is loose or switch contact is poor.
9.The fuse box is fused.
10.The "auto-manual-de-excitation" switch on the control panel is in the de-excitation position (automatic generator set has "auto-manual-de-excitation" switch).
11.The voltmeter is damaged.
12.Rotary rectifier module is damaged.

One of these manuals may help https://www.masterspas.com/hot-tub-owners-manuals

Your RV and others have a "Transfer Switch"...
Follow the exterior cable coming from the electrical INLET mounted on the outside of the RV.
It should go to a junction box, or the actual transfer switch.
May have the problem there... Bad Contactor or the Electronics are not seeing the Generator...

• The completely automatic transfer switch monitors incoming voltage from the utility line, around the clock.
• When utility power is interrupted, the automatic transfer switch immediately senses the problem and signals the generator to start.
• Once the generator is running at proper speed, the automatic transfer switch safely shuts off the utility line and simultaneously opens the generator power line from the generator.
• Within seconds, your generator system begins supplying electricity to the critical emergency circuits of your home or business. The transfer switch continues to monitor the utility line conditions.
• When the automatic transfer switch senses the utility line voltage has returned at a steady state, it re-transfers the electrical load back to the utility line and resumes monitoring for subsequent utility loss. The generator will continue to run for an engine cool-down period of several minutes while the entire system stands ready for the next power outage.

https://www.dieselgeneratortech.com/generator-sets/Information-of-Starting-Battery-in-Diesel-Generator-Set.html

[PDF]Onan Master Service Manual.pdf

michaelstractors.com/forum/.../Onan%20Master%20Service%20Manual....
Jun 28, 1978 - rated at seventeen and one half (17.5) kilowatts or less. ... of an Onanengine generator set, automatic transfer switch, running time meter and .

Check for a bad fuel pump and a clogged fuel filter.

What do you mean won't swap over to 120Volts? are you referring to a transfer switch not switching over to generator?

What is the Spec number?
Also can you read a fault code on the start switch?
Do you know how to read the codes and post? if not let me know

You should not have to switch between battery and generator, unless you are trying to use an inverter as well. Batteries are 12vdc, typically run lighting, water pump, furnace, fridge controls. Generator powers ac, microwave, television, and anything else that plugs into a 120vac wall outlet. Inverter may also run select loads, and possibly dedicated wall outlets, but not AC, fridge, or microwave (usually).

Most RVs / Motorhomes now have automatic transfer switches that will switch between the generator, shorepower, and possibly an inverter. Your coach is made during an era of transition from manual transfer switch (in form of heavy duty electrical outlet near shore power cord) and the automatic transfer switch.

There is no specific place where a transfer switch or generator receptacle are mounted on an RV. There are no rules or regulations tha cover this as far as I know. Your RV may also have an EMS (Energy Management System) that will control the transfer switch as well. EMS panels are typically located above the refrigerator or above driver / passenger seats in overhead compartment.

The voltage regulator (AVR ) already faulty.