Use boxes like Makpac boxes instead of bags wherever possible. Make up your own partitions to separate tools using 9mm plywood with foam protection at bottom and sides as needed. Tools floating around in bags or loose in the back of the van get dinged and damaged quite quickly as well as lost; ply lining gives you somewhere to clip or store certain tools such as skeleton guns and so forth (Google "Terry clips", etc). The system boxes allow you to select only the kits you require for the day if organised properly. Consider a matching open tote for loose tools, where appropriate, and for awkward shaped tools like caulking guns which won't always fit in standard boxes - these box systems allow you to clip together boxes an carry 4 to 6 tool sets at the same time (much easier than struggling with multiple differently sized manufacturers blow moulded cases) and all the major power tool makers have them (I use a mixture of Systainers, Makpacs, Hikoki HSC and Metabo Metaboxes as they are all compatible but I still have several Stanley pull along boxes in use as well). Add a piece of VCR paper inside each box. Add a silica gel bag, too, and hang a large one up inside the van all the time (can be had from Amazon, etc) - silica gel bags can be dried and reused. Spray everything with WD40 or wipe down with a rag dampened with 3 in 1 oil or sewing machine oil every time you put something metal away (including steel drill chucks steel tool holders on jig saws, impact drivers, etc). OK, so you'll have to wipe tools off when you come to use them from time to time, but better that than rust! Only carry tools you need for the job - that may mean having separate boxes for a door hanging kit (trimmer, planer, drill, etc), plumbing, as well as a couple of general boxes containing stuff you almost always use. For me, at the moment, that is hammer, nippers, cordless drill, drill bits, impact driver, pry bar, cat's paw, nail sets, Stanley knife, tape measure, etc. This list changes over time
Ply lining a van seems to make it drier as there is less cold surface for water to condense onto.