For most tires the Speed Symbol letter should come right after the size numbers as part of the "Service Description". There will also probably be a "Load Index" number like 118 or similar range depending on tire size and Load Range.
The tire in post #17
shows the normal location and for that tire the Load Index is 118 and the Speed Symbol is "N".
The Tire Rack page covers this information very well.
I note that under the new requirements for ST type tires some companies have decided to not use the Service Description as seen on P-type tires but to apply a mph number such as 75mph. As I understand the regulations this is their option. An example of the alternate marking is seen in the picture in post #34
Side note. The speed rating issue for ST tires came about with action on "dumping" of tires below cost so restrictions and duty was placed on some tires. ST type were given an exception. The addition of speed symbol on ST tires started to show up in Dec 2015 so it is very unlikely that if your tires were made much before March 2016 that they have a statement on speed or a speed symbol.
I write a blog on RV tire application and safety. RVTireSafety.com
Also give seminars on tires at RV events across the US. 40 years experience as tire design & quality engineer for major tire mfg. Freelander 23QB on Chevy chassis is my RV