1) engine belts
2) multi-purpose relays & solenoids in capacities likely to be used on your rig (30, 40 & 50A generic Bosch type relays, 5 or more of the 30's which are ubiquitous on most rigs; Cole Hersee #24059 makes a good generic solenoid buy from RyderFleetProducts.com; may also need spare charging solenoid if rig is so equipped)
3) Mulit-meters:
...a) good digital, I have a Fluke87 which is overkill but it works, Craftsman has a couple of good ones, expect to pay near $100
...b) RadioShack analog- sometimes the needle is better than the numeral readout (flashing circuits) or I need a second, expect to pay $25
...c) Power Probe III (google) which is rapidly becoming my favorite because of its excellent diagnostic options; also the piercing probe ends (I bought the kit that has two sizes of piercing type) are the best I've seen, good kit ~$150. Great for finding a short or powering a low amp 12V device to test it, has tones for 12V & ground, slick gizmo
4) 36"x175# zip ties, 14" standard zip ties (buy the latter @ HD or Lowes in the 500 count bag- you'll use them like they only cost 3 cents each, cuz they do, and if you buy them in this quantity you don't feel bad about not reaching for a shorter one- just cut off the excess (I'm not really sure why they even make 7" and 11")
5) stainless steel tie wire (Harbor Freight)
6) assortment of spade, ring, & butt press type wire terminals, w/a
good electrical pliers that has cutters and a high quality press form; I buy the good terminals @ my local
Fastenal, they carry quality stuff. Wire nuts assorted sizes, dozen each size incl. the bitty ones which are really handy. 18, 14 12, & 10ga wire, 25' of each should get you thru most emergencies, black is fine if you deal w/the color when you get home or somewhere you can do a proper color-coded job (you can buy blue, yellow, white & purple electrical tape & wrap black wire @ ends & exposed places so your first repair is permanent). If you are the meticulous sort, go to Uline.com and order the 1000ct bag of S-7747 "ID Ties" and take an ultra-fine Sharpie permanent marker; when you encounter a circuit that was a PITA to isolate, wrap the wire & mark its circuit on the tag (after you've been at this for a while, and you re-visit a marked circuit you will see the genius of it).
7) X-Treme tape, Scotch brand electrical tape in black, red & green (w/the cheap brands the stickup cooks in the hot sun and makes a snotty mess, Scotch's stickup holds up better; get the big roll in the plastic tupperware-like container to keep it from attracting dirt)
8) motor & trans oil, coolant, distilled water for topping off machinery & batteries (or go AGMs & forget the distilled water), chassis hydraulic oil if rig is so equipped
9) spare batteries for flashlights, FRS radios, clocks, smoke detector, blah, blah, blah
10) water filters
11) plug ends for vital electrical cords (sooner or later you will meet a pedestal that heats up & ruins your plug end & you'll wish you had a ready spare
12) house water pump (I have mine installed w/flex lines ready to plumb/wire over when the main pump chokes; only needed it once so far, but went coupla weeks on the spare no problem & replaced the main at a good price when a bargain presented; spare doesn't have to be high dollar, just needs to pump when you want water)
13) fuses of every description used on your rig, fuse tester (GM type can be bought in a kit w/a lotta GM fuses) or use your MM or Power Probe
15) spare bulbs for all lights; HWH (or PowerGear or other) o-rings for hydraulic slide solenoid valves; spare solenoid valve
16) dielectric grease, JB Weld, non-JB epoxy, red Loctite, blue Loctite, contact cement, light machine oil in pin-point oiler, WD-40 or eq, molybdenum grease in a squeeze tube, NSF approved silicone grease, Anti-seize (I carry a dry stick type for threads which is less messy than liquid, & a brush-applied paste type w/copper shavings in it for electrical connections that need corrosion protection)
17) OBD2 engine code reader/analyzer, coach engine computer software/laptop (gettin a little spendy here, so this is optional unless you are traveling in Mexico where a full breakdown is going to be a weeks-long ordeal otherwise)
18) flange nuts from 1/4-20 to 5/8-11 (more universal than regular nuts), jamb nuts in same size ranges, lock nuts in same sizes, 3/8 1/2 and 5/8 coupler nuts if you plan any remodeling in the basement (in which case some 3/8 and 1/2 allthread also)
19) spare hitch pin (I carry 5/8-11 x 5" bolt as it can double for other uses)
That oughta get you started ;-)