It doesn't sound like a smart move to me. You're basically trading a possible loss of value for a guaranteed loss of value.
The MH MAY lose value in the future - IF you decide to sell it AND IF enough potential buyers would be deterred due to the age to make it unsellable.
The 5er AND the truck WILL lose value as you drive off the lot. That's a definite loss right now.
In addition, if you trade in the MH, you will not get its full resale value but a lower value. The value to trade in price ratio now will most likely be worse than the the value to trade in price ratio in 5 to 10 years.
Looking at your numbers, you didn't buy the MH new. How long have you had it?
Some questions to think about: How long do you plan on continuing to RV? Are you happy with the MH (other than the concern about the engine sitting)? Does it meet your RVing needs? Do you expect any change to your RVing needs in the next 5 to 10 years?
1975 American Clipper RV with Dodge 360 (photo in profile)
1998 American Clipper Fold n Roll Folding Trailer
Both born in Morgan Hill, CA to Irv Perch (Daddy of the Aristocrat trailers)