I live in the Phoenix area.
Phoenix, Mesa, Casa Grande, Tucson are all places that have plenty of RV campgrounds. Spend a month or two may be a problem as everything fills up for the winter quickly as the snowbirds arrive in November and don't leave until end of March or April.
Spending a month depends on what you like to do.
If you are coming in from southern California heading back east these are some suggestions to stop and enjoy.
Las Vegas NV - Hoover Dam
Williams AZ - I-40 to the Grand Canyon Railway RV Park. Great campground and you can ride the railroad to see the south rim of the Grand Canyon. You can drive but the RR is a great day trip with bus tours of the sites along the south rim.
RV Park Location | Grand Canyon Railway & Hotel
Sedona AZ is a beautiful stop over.
Red Rock State Park near Sedona has a nice campground
Tucson AZ has lots to see and do. Old Tombstone and other western stuff.
Suggestion for campground if you are over 55.
Ricon Country West Campground just south of Tucson off of I-10
From there east you have not much until you hit El Paso and San Antonio Texas.
Now you can head north to
Albuquerque NM for the balloon festival in Oct I think.
Then to Amarillo TX to Oklahoma City and I guess you pick your way home.
So this give you a few suggestions as I have Rv and stayed many places on that route.
If you want spend the two months like you said I would suggest Palm Spring Ca if you can get in.
RV Resort in Mesa AZ east of Phoenix is my next choice but those places fill up quickly. They do have lots to do at those Mesa Resorts and you are still close to Phoenix.
Hope this helps you a bit.
If you need more details just ask.
Iggy