I've always used a bit of moisture on mine too, but more often than not, they fall off several days, or even 3-4 weeks later. I've ensured the glass is clean and the cup is clean.
It's very disconcerting when they do fall, usually at the most inopportune times too. I'll still use the suction cup in the Canyon as I don't have a permanent mount for it. The vast majority of use for the car GPS is in the motorhome, however.
Forest River installed the Jensen unit in the dash which includes a GPS, but it is very hard to read and interact with. It appears they laid out the typefaces & line weights on a 30" screen where things appeared huge, but when it was sized down to the double-din head unit... the result was less than useful.