While there are a number of considerations that will significantly change the value of any RV, most banks use NADA or a similar service to estimate the loan value of an RV. For that purpose, they usually require the RV to be in serviceable, operable condition without any significant problems.
The website for 2005 Keystone models is:
http://www.nadaguides.com/RVs/2005/Used/Keystone-RV
The NADA website indicates that the "average" retail for a 2005 Zepplin 242 is $5450 and the "average" retail for a 241 is $7300.
While these are just "starting point" values, much more should be considered such as damage, broken/missing components, tire/axle/brake condition as well as general overall appearance and condition.
I've always heard that "one man's treasure is another man's junk" so what it would be worth to you may not be the same as it would be to anyone else who may be looking at the same RV.
Good Luck,