5) Axles. 1/2-ton chevy axles (10 bolts on differential cover) quickly become a weak link if you get into large tires and trail abuse. Until you get to that point, I wouldn't worry too much about changing them. Though it is worth noting that factory positraction carriers have a habit of fragmenting and wrecking differentials. This can happen under fairly normal driving habits. Not something to freak out about, but if you are driving such a unit (as I am), you should have a plan for replacing it eventually. Best way to check gear ratios is to pull the cover off the differential, but you can check them from the outside. If you still have the RPO tag in the glovebox, it will tell you what options the truck came with, including axle ratio and whether you have the dreaded positraction unit.
3/4-ton trucks (after about 1980) have a somewhat sturdier 14-bolt semi-float axle (basically your axle scaled up a little). Older 3/4-ton trucks and 1-ton trucks have a 14-bolt full-float axle. These have advantages (easier maintenance, much sturdier design, etc), and that is probably what your friend is assuming you'll find underneath that motorhome (although it could also be something else). Are those dual tires underneath there? That would be a headache. Also, those are 8-lug wheels, while your truck is set up for 6-lug wheels. So if you swap, you're stuck changing the lug pattern in the front as well (not hard, but it's extra work). And swapping in an axle, it is important that the gear ratios match between the front and rear axles. Having wheels spin at different speeds causes some interesting things to happen.
6) You really, really don't want the suspension from that. Taking springs off of a 12,000 pound motorhome and putting them on your 5,000 pound will yield a
very stiff ride with very little wheel travel. Not any improvement over what you already have.