Are they necessary, not really..
On my first two MHs, I carried various lengths of 2x6 ( 12 - 24 inch). I could usually get the MH reasonably level without putting any of the 2x6 under tires..
With haveing a slide on our Southwind, the jacks are more of a "stabilizer" than a leveling device. they help stabilize and bouncing around, especially when that slide is sticking way out there.
My Southwind has four jacks, but its really a tripod system. The two front jacks are plumbed together and serve as one jack. The two rear jacks are separate and make up the other two legs for stabilizing.
I don't think it would be worth the expense to add them to your MH.
1999 Fleetwood Southwind 35S (Ford F53 6.8L V10 275hp 4R100 4 spd trans) - Toad 2003 Saturn Vue.
It won't do MACH 2, but I can get a sandwich and take a pee.