We went from a pop-up to a 35' class A. Used that one for a few years and upgraded to a 38' Newmar.
With your schedule it appears that a diesel would give you no return for the extra cost of just owning one as you plan to allow it to sit much of the year. A diesel sitting is no better than a gas sitting...its all just metal siting in storage.
For your budget you can get a much newer gas powered MH and the engines will give you all the service you will need. Look for late model Newmars on the Ford 24,000-26,000 lb. chassis with 22.5 wheels they have 5 speed transmission with Tow Haul.. Excellent quality built home.