Just from memory and not actually standing at my trailer this early in the morning, I seem to recall that these bolts are standard 1/2 inch bolts about 1 inch long. If I were in this situation, I would fo to ACE hardware and get a sampling of different sizes with standard threads - stainless, of course. Try them and don't forget to lube the old threads. If you have a tap and die set, you could use the thread gauge to get the number of threads per inch and then retap the hoole to get the bolt size.
This would be easier if you could get the trailer to the store. Either way, this is one of the simpler problems to repair. Hopefully someone will pitch in and help. The repair manual for my '73 Excella only says remove the bolts and resuse to install a new jack - no picture or description.
1973 Excella 500