Is sitting for four months detrimental to a MH?

x3 that it should not be a problem. Up here where we have this thing called "winter" many, many people leave their motorhomes sitting unmoved for a few months every year.

If you don't have them, I would highly suggest getting vent covers for at least one or two of the vents and leaving them cracked open slightly while it's parked to give some ventilation. Vent covers are just generally really great things to have. As a trailer owner, that's probably all old hat to you anyhow.