GaryM,
Sorry, been out of town for 4 days, just catching up.
<font color="blue"> Can I make a pigtail and box with two 110V outlets and feed it from this outlet </font color>
Don't do this unless you can put some sort of 20A protection on it.
<font color="blue"> My wirefeed welder wants 20A </font color>
You can get 20A devices at the box stores, Lowes, Home Depot. Just replace one of your current 15A ones, provided you have a 20A breaker on it.
<font color="blue"> Could I get 50A at 110V to feed a friends RV if they came by to stay for a while</font color>
Yes you can make a pigtail to go from 4-wire 50A 220V to 3-wire 110V. If they make a 50A 110V receptacle. I've used 30A. maybe they make 50 as well. Make sure it is marked RV outlet.