\documentclass{article}
\makeatletter

\newcommand{\ifblank}[1]{% from expl3
  \expandafter\ifx\expandafter\relax\detokenize\expandafter{\@gobble#1?}\relax
    \expandafter\@firstoftwo
  \else
    \expandafter\@secondoftwo
  \fi}

\def\etb@rti@prs#1#2{%
  \ifx&#1%
    \expandafter\@firstoftwo
  \else
    \expandafter\@secondoftwo
  \fi
  {#1#2}
  {\ifx&#2%
     \expandafter\@firstoftwo
   \else
     \expandafter\@secondoftwo
   \fi
   {\etb@rti@chk#1+\etb@rti@num#1#2}
   {\etb@rti@chk#1\etb@rti@chk#2%
    \ifnum\etb@rti@num#1<\etb@rti@num#2 %
      \expandafter\@firstoftwo
    \else
      \expandafter\@secondoftwo
    \fi
    {+\etb@rti@num#2-\etb@rti@num#1\etb@rti@prs}
    {+\etb@rti@num#1\etb@rti@prs#2}}}}

\def\etb@rti@chk#1{%
  \ifcsname etb@rmn@#1\endcsname
  \else
    \expandafter\etb@rti@brk
  \fi}

\def\etb@rti@brk#1&{+\z@&-1}
\def\etb@rti@end#1&#2\relax{\ifblank{#2}{#1}{#2}}
\def\etb@rti@num#1{\csname etb@rmn@#1\endcsname}

\chardef\etb@rmn@i=1
\chardef\etb@rmn@I=1
\chardef\etb@rmn@v=5
\chardef\etb@rmn@V=5
\chardef\etb@rmn@x=10
\chardef\etb@rmn@X=10
\chardef\etb@rmn@l=50
\chardef\etb@rmn@L=50
\chardef\etb@rmn@c=100
\chardef\etb@rmn@C=100
\mathchardef\etb@rmn@d=500
\mathchardef\etb@rmn@D=500
\mathchardef\etb@rmn@m=1000
\mathchardef\etb@rmn@M=1000


\begin{document}

% --- example
\expandafter\etb@rti@end\number\numexpr\expandafter\etb@rti@prs\detokenize{mcmxcv}&\relax

\end{document}