My tranny doesn't work in D but 1st 2nd and reverse are fine how fast can I drive in 2nd gear I have a clss A motorhome

I can't give you an exact number, but typically 2nd gear is going to get you to about 30mph. Pushing faster than that in 2nd for long distances could potentially damage your engine.

You may have a transmission problem or an issue with your gear change linkage, neither of which you are likely to be able to look into yourself. One thing you could and should check is the transmission fluid level, if it is low fill it up and see if that gets it working again, then monitor how quickly you lose it. If it's a slow leak you may be able to live with it, although I'd still get it checked out.