If the work is very good, it may increase the resale slightly, though usually nowhere near enough to be profitable.  If the work is ho-hum or worse, it may decrease the resale value some, of course depending on what the condition is originally.
By and large, though, I wouldn't expect a vast change in value for a 20 year old motorhome.  The value is rather limited to begin with due to the age.  I generally agree with Dave that you'd be best served by doing what works and appeals to your sensibilities and not worry about the affect on resale value.