A good crimper.... Well for F-Connectors I don't have one yet but for regular wire splices... Mine is professional grade, does a fantastic job, I've seen factory crimps that were not as good.
Electrial meters VOM are handy on occasion,,, Mine also does a few other things like temperture and frequency (it is a very nice Radio Shack closed out model, I suspect made by FLUKE due to the case it fits like a hand fits a glove)
That is one tool I've found many RVers do not have, Even a cheap one can be very useful (though less accurate) and yes, I have purchased them at a dollar store.
"Goo" (a general purpose sealant/adheasive) finds many uses too. There is an old saying that you really only need two things... Duct Tape and WD-40. If it moves and it should not, Duct Tape. If it should move and it does not, WD-40. Sometimes, however, tape is not the best, then GOO works, Comes in a lot of flavors, Shoe Goo, Automotive, Plumbers, Household.. More or less all the same product, all interchangable