Need advice on ways to grow these. Have tried to grow before but the melons only get so big then they rot on vines.

I plant mine in hills with approx. 4 plants per hill. I first work in lots of cow manure, compost, whatever, into each hill. I usually plant 7 or 8 seeds per hill and thin the the best four. I put a thick layer of wheat straw for mulch well out around each hill so the melons will have something between them and the soil. Keep them moist. Not sure what's causing your problems with rot--maybe the wheat straw will help with that. If the melons get too much hot sun, they will rot quicker than normal. I believe you folks up there in OK get pretty hot too. When it gets to the real hot part of summer here in MS, I pick my melons a little early, lay them out and cover with an old sheet or something to let them finish ripening.

I read somewhere to keep them off of the dirt especially if its moist and to put straw under them or put them up on an old coffee can.

Placing something under the melons will help greatly reduce the rot, even newspaper or cardboard would help, but straw works best.

i've seen posts of people here that put ceramic tiles underneath the melons. I actually left mine on bare dirt and turned them daily just a 1/4 or 1/2 a turn here & there. but i did tha tevery day!

For those who like to can their garden goodies (like me) there is a much cheaper method. I save my used canning seals and toward the end of the season when my straw is beginning to break down, I just slip a canning seal under the developing melons--works like a charm. By late in the season when the vines are extensive, it's kinda hard to get in there to add more straw.

What you have sounds like BER for Watermelon's..
**Blossom End Rot**

BER or Blossom End Rot is caused by a lack of calcium in the developing fruit.

This can be from either it not being in the soil and/or available to the plant to absorb..

Or when the watering is inconsistant..
Like tooo much water, then a dry spell, irregular watering habit's or practice's or naturally accuring with or without rain...

You can use Dolamite Lime *to sweeten the soil*
***Have you done a soil test to see how your soil is though?????***
Or you can spread/sprinkle some Epsom Salts in the soil and on top of soil, time of planting..
Or even spray of Epsom Salts too on the foliage...