Hi dougherty771,
Welcome to the Heartland Owners Forum.
Some of the characters dropped out in your message, so I'm guessing a bit as to what you have.
But if the shore power cord has a plug with 4 pins, it's 50 amp. 3 pins is 30 amp.
50 amp trailers plug into a standard NEMA 14-50R receptacle which is wired like a 240V oven receptacle in your house. There are 2 hot legs used independently inside the trailer. So everything inside the trailer is 120V AC (other than the 12V DC stuff).
30 amp trailers plug into a NEMA TT-30R receptacle which is specifically designed and wired for RVs. It is 120V only. Although it looks a little like a 30 amp receptacle for a household dryer, the household dryer receptacle is wired for 240V. It's unfortunately quite common for electricians to take a TT-30R receptacle and wire it for 240V by mistake, because they're used to dryer receptacles. If you plug into 240V, you will cause quite a bit of damage to your trailer.
Take a look at our owner-written
Electrical User Guide. It explains all this in some detail with pictures and drawings to help make it clear.
50 amp service normally uses awg 6 wire, but 125' is a pretty long wire run. Depending on what type of wire you use and whether enclosed in conduit, you might be able to run awg 6. If you're not sure, you should probably get advice from a licensed electrician on the right wire for your situation.