Good to hear from you, sorry I can’t help with your display issue as I don’t have your version of MoHo. But if you have drained down your waste tank and still have 4 bars showing then perhaps either drain your fresh water as well and see if that affects the display to give you a clue on which is which. I expect you have a faulty sensor in your waste if the display still shows 4 bars after trying the above. So probably a warranty job for the dealer.
Regarding the handbook issue, I know Hobby have many of them on their website but as you say yours is greyed out. You may be lucky if one of our group with a 2011 750 reads this and can send you a scan. Alternatively, are you a member of the Hobby Motorhome Enthusiasts Facebook site or our Club Facebook site. I am sure somebody will have the same model and can explain the display setup for you.
Hope to see you again on on of our rallies.