Is sitting for four months detrimental to a MH?

Not at all. If you don't have electrical hookup you'll need to disconnect the batteries or charge every weekend. Shut the power & water down when gone. You could do solar depending on how many batteries you have.Keep battery fluids full.

Ours sits for 6-7 months every winter, plugged into power.Just add a fuel stabilizer.Being in Texas you might want to add some water collectors to keep the humidity out of the coach.