Can anybody point me to information on how to modify RandomItem.xml?

<ITEM>
     <uiIndex>2748</uiIndex>
     <szItemName>Rnd Misc Item 1</szItemName>
     <szLongItemName>Rnd Face Item</szLongItemName>
     <usItemClass>1073741824</usItemClass>
     <randomitem>7</randomitem>
     ...
</ITEM>

<RANDOMITEM>
     <uiIndex>7</uiIndex>
     <szName>Random Face Item, complete</szName>
     <randomitem1>1</randomitem1>
     ...
     <randomitem10>6</randomitem10>
     <item1>0</item1>
     ...
     <item10>0</item10>
</RANDOMITEM>

<RANDOMITEM>
     <uiIndex>1</uiIndex>
     <szName>Random Face Item, Group 1/6</szName>
     <randomitem1>0</randomitem1>
     ...      
     <randomitem10>0</randomitem10>
     <item1>176</item1>
     ...
     <item10>2306</item10>
</RANDOMITEM>

<ITEM>
     <uiIndex>176</uiIndex>
     <szItemName>Steel Helmet</szItemName>
     <szLongItemName>Steel Helmet</szLongItemName>
     <usItemClass>2048</usItemClass>
     ...
</ITEM>

what are the properties of <RandomItem1-10> tag and of <Item1-10> tag.

Thanks for trying. I still don't understand the most important part - which is #3

1) If you enter items only in the tags <itemX> XX </ itemX>, then only the item <item1> X </item1> will always be taken.
2) If the tags contain <randomitemX> and <itemX> tags, then the selection will occur only between <randomitem>.
3) For correct operation, you need to create a separate object <RANDOMITEM XX>, which contains items from Items.xml, and separately, an object <RANDOMITEM YY>, which refers to an object <RANDOMITEM XX> through the tag <randomitemХ> ХХ </ randomitemХ> .

That means - don't mix itemXX and randomitemXX tags in one record in RandomItem.xml, if you need only 10 possible random items, just make one record with only ItemXX tags, if you need more random items, you have to use nested tree in which case you create one record with only RandomItemXX tags which are ID references to the records in the same file where you can either define itemXX tags so the game translates it to real items or add another level of random items in which case you only use RandomItemXX tags.

Does it happen too with default 1.13 ja2.exe?