Diary of a Wimpy Kid: The Long Haul (2017)

Answer: "Beardo" looked intimidating and could easily frighten a child (or adult, for that matter). He also became enraged in front of children over scratches on his van - something minor that might bother an adult, but certainly not serious enough to produce Beardo's over-reaction. Greg did deny that he caused the damage. However, Beardo did not come across as being a rational adult or someone who a 12-year-old would feel comfortable standing up to or simply trying to reason with. Greg saw Beardo as a "difficult" and irrational person, so rightfully wanted to get away from him as soon as possible.

KeyZOid

Answer: Mainly because when they first actually meet, the Beardo's daughter blames everything on Greg, so even if Greg did try to explain everything, the Beardo probably wouldn't believe him.

Answer: There's probably a combination of factors why Greg kept things to himself. Teenagers don't tell parents a lot of things for whatever reasons (e.g, not worth the hassle). Maybe Greg thought his parents wouldn't believe his side of the story and assume he was the instigator. Perhaps Greg's parents gave the boys a stern warning to behave themselves while on vacation - that they need/ want rest and relaxation or care-free days without trouble, especially from the boys (trying to avoid the "can't you even behave yourselves on vacation" lecture). Believing "ignorance is bliss", Greg figured what his parents didn't know couldn't hurt them. Telling his parents may not have been helpful or relevant; Beardo was acting like a lunatic or psycho on each encounter. It might have been difficult, if not impossible, to tell his parents without Rodrick overhearing. Some of the things Greg did (getting into bed with Beardo) were embarrassing; if Rodrick knew, they'd become the brunt of his teasing.

KeyZOid