Thanks!

With the groups help I was able to figure out what I was doing wrong.

As suggested I tested the main lugs in the subpanel I got 220v, but did
not get 220v when I tested the outlet accross both hots.

Turns out the the sub-panel installed only has two positions that will
give you 220v. I didn't connect the breaker to either one of these. I
removed the breaker, and re-connected it to the correct lugs and got
220v accross both hots. The heater now works! The inside of the panel
door has a diagram that shows which lugs are 120v and which are 220v.

Thanks for all your help!