Xantrex 458

Are you on shore power or boondocking? You are either using more power than your batts can supply overnight (furnace, lights, etc) and not recharging them enough during the day, or the batteries are completely shot - or both. If you were on shore power you likely would not be having these issues. The fact that the gen won't start - which requires a decent charge from the batteries, tells me you are draining them to the point they won't support your demand. You either need new batteries, more batteries, a more steady rate of recharge (solar, shore power, generator) or all of the above. Also sounds like you need better power management. The title of the post is about a Xantrex converter which converts battery power to 110. But it doesn't create power if the batteries are dead. You need to know how many amp hours your batts will supply and keep your demand lower than that. You mention the lights are dim even when you plug into shore power. The lights always run on the batteries and shore power charges the batteries so the fact they are dim is not surprising if the batteries are low. If you have access to shore power why are you unplugging it?

I suggest you familiarize yourself with what runs on 12V, what runs on 120, amp hours and power management. 

This may help:

The 12volt Side of Life (Part 1)

And the buttons on your GFCI wont do anything because there is no power to the GFCI

It's winter so I assume your heater is working overtime. Yes the heater is on propane, but the blower fan for the heat is probably the largest 12v power hog in the system. What is your thermostat set at and how often do you hear the furnace coming on at night? People typically report getting only one night of power per installed battery when using a lot of heat. 

When you say the lights are dim, even when you plug into 120v; Where is the 120v coming from? Are you referring to 120v when the generator is running? Is it from plugging into the inverter? Or from the shore power cord plugged into utility power like your garage, or maybe power post in a campground?

If you are plugging your shore power cord into utility power and your lights are still dim then there power is not getting from the power cord into the RV. Try checking all the breakers and make sure they are all on. If you're not sure then unplug the power cord and turn off the generator. Then flip all the breakers to off. Push extra firmly to the off direction to make sure they are all off.

Now with all the breakers still off plug back into shore power. Then flip all the breakers back on starting with the main breaker. Do your lights get brighter and do the 120v items work? Check things like a fan plugged into the wall or the microwave or air conditioner fan. Do the 120v items work?

I’m sorry to hear about your situation. As you’re really unprepared to deal with RV maintenance and repairs. I hope your situation is voluntary. 
That said, it is about impossible to diagnose your issue as you described a whole bunch of things basically as one not too descriptive run on sentence. 
Best bet is first verify whatever source you’re using to charge the batteries is working. Next test if the batteries are actually good. 
from there, idk. You could have a multitude of pre existing issues or one issue like bad batteries causing the whole mess. 
please try to isolate what happens and when and in what order. 

Thank you every one that took time to read and reply to my situation it's much appreciated.  No this wasn't really voluntary live situation it's what it has to be at the moment. I don't know any thing about this stuff I've never even been inside a motor home like this before. It is a very nice motor home don't get me wrong but very out dated for it's kind as well.  Still super new to me.  It wasn't my choice of purchase but it is what I got.  I'm pretty sure it sat for a period of time without batteries being recharged periodically so I'm sure they are shot and the xantrex box in pretty sure it was going out and now it's done.  From what all of u have told me and some videos I've watched and other posts from other people.  And I can't afford another box rt now although I did just get 3 hand me down batteries that were properly taken care of and a good couple size solar panels and a control box for solar I'm not sure how good that box is but none of it is hooked up yet.  I've been using flashlights at night for now.  I've gotta get it all fixed some how.  The 3 batteries I just got weigh like 150lbs each . I have 6 batteries in here rt now 2 are too run the motor home engine which that works it can be driven just the coach stuff doesn't work which there are 4 batteries currently in there for that.  Thank you again every one.