Hugs, you can do this! Might be a little stressful but it will be good if you face it with a positive outlook.
The first thing I thought of was Granddad's needs... any special equipment? walker? wheelchair? bed? bedside poty? oxygen?... gotta make room for that and find places that are easily accessable to him and any folks that take care of those equipment things...
Which room will be his? start there get it cleaned out and set up.
His new room will mean you guys loosing room so who's doubling up and in what area? tackle that next.
I'm assuming your school age kids are bigger, let them help you do most of the work with you directing if they are big enough. Get the extended family involved also in the moving of furnature and boxes from one room to the next.
What about long term storage of things like out of season clothes and decorations, can you ask that one of the extended family store them for you to give you more room?
My first thought was to build shelves high on the walls up near the ceiling for storage, don't know how/what would need to be done with the mobile home to hang that weight, in my brother's old m.h. he had issues with the size of boards used in the walls for support. He was able to hang shelves and such but he had to put extra support brackets and beef the shelving up some.
If you can raise the beds off the floor more there's added bonus space (might not want to go too high with granddads or yours with you being pregnant and possibly a babe co-sleeping... don't need to be climbing a step stool!
dishes, go down to the bare minimum needed for everyone to have a setting and you don't have to take shifts in eating because you only have 1 spoon, makes for having to keep up with washing dishes but you gain a little cabinet space for food and sale storage. Towels and sheets, pare them down too, might want to keep a couple extra sets of sheets for nighttime messies but you really don't need 4 sets for each bed. Towels, 1 set for each person and then a couple for messies and if you guys do the outdoor water thing in the summer then enough to make it so that when the kids drag the nice ones out in the mud you don't pull your hair out.
Farm/ranch... just saw that... do you have storage sheds/barns? could you set up tubs or cabinets for used but not used often enough to keep out there?
I'm thinking along the lines of a utililatrian sweep, if it's just pretty to look at then box it up and get it out... that is a massive declutter and downsizing and is a whole lot easier said than done.
The new baby? do you have things set up for it's arrival? Have you started nesting and making space for the new addition? Need to plan space and time for this too.
Care for granddad? does he need extra? can you get help from family or other places? Ask relatives if you can to help with making some meals to freeze up or ones that you can mix and cook up quickly.
You really can do this.