I was a boater on the St. Croix for most of my life. There are a few launches in that area (Hudson Public Launch, Beanies Marina across the bridge in MN). I like the Hudson public launch better becasue it is easier to park the trailer and it is a little cheaper. It is just north of HWY 94. It is the last exit before reaching MN.
As for Gas youhave many options
In Hudson there is a marina just south of the launch that has gas
Stillwater,MN 10 miles north of Hudson - Gas is located just north of the draw bridge on the right hand side. Next to the bar called P.D. Pappy's
Afton, MN 10 miles south of Hudson at marina inside no-wake zone
Prescott, WI 20-25 miles south of Hudson inside no-wake zone
Red Wing, MN South of lock #3 40 miles south of Hudson
As for places to stop,there are many beaches to pull up to in the area. There are a few good bars in Stillwater (freight house, PD pappy's) and Hudson (don't remember the names) You can also boat up to the casino called Treasure Island. It is located just north of Lock #3 in Red Wing about 35 miles south of Hudson.
Hope this helps. Any questions let me know