Bathing is not the same as washing. People washed themselves regularly usually by wiping down with wet and scented towels, and buckets of water. Those who lived near a body of water or streams would swim in there as well.

Almost no one had a bathtub to bathe in and in the Middle Ages bath houses were seen as places of prostitution so they stopped building them.

>but does this "myth" have any merit?

In the middle ages people had a good understanding of basic hygiene, in short immersing yourself in water (warm if you can afford it) is good. The Romans had bath after all, it's nothing groundbreaking. And yes there were many bathhouses in medieval European cities which served both as bath houses and brothels.

The "dirty era" is actually the modern period (16th-18th centuries). Because bath were seen as dangerous: doctors thought that pores would open in the bath and let enter all the diseases. That's where you see stories of "a man proud of never having taken a bath in his life" and actual doctor saying that washing anything more than the hands and face is unhealthy. There's a reason why everybody stank at Louis XIV's court.

I think it's dependable on the period and area. The Vikings (or more properly Scandinavians) were notorious for their good hygiene, which, kind of, put to shame the Anglo-Saxons who bathed much rarer. In the 11th century, an otherwise unknown Trotula of Salerno wrote "De ornatu mulierum" a detailed guidebook for proper hygiene of women. John, King of England was known to take a tub along with a "tub-ward" on his journeys. In Poland of High Middle Ages public baths were very common and cheap, and Polish King Władysław Jagiełło (r. 1386-1434) kept a very high hygiene - even receipts for repairs and extensions to the baths he used have survived. Finally, there survived several treatises detailing bathing techniques, such as "Magninius Mediolanesis" and "Regimen Sanitatis Salernitanum" - both from Apenine Penninsula from 13th and 14th century respectively.

If by bathing you mean "fulling immersing yourself in water", then yes, most people would only bathe once a week. Keep in mind that most people did not have running water in their homes until around the mid-19th century so taking a full bath in the past often meant have to haul multiple buckets of water.

That being said, though, it was very common to take sort of sponge baths everyday by washing the face, hands, feet, armpits, and genitals. (I did that recently when our pipes got filled with resin from a malfunctioning water softener and I found that I stayed surprisingly clean even without taking a full shower.) Also, it's important to remember that handwashing before meals was a must throughout pre-modern Asia and Europe.

The real problem was not so much lack of bathing as a difficulty keeping water clean. That had also been a problem in the ancient Roman public baths; the water in them was not chlorinated like modern swimming pools are, which meant so rather than preventing disease, they seem to have been very likely to given visitors internal parasites.

Theres a good quote in Monty Pythons the Holy Grail:

"He must be a king."

"Why?"

"He hasn't got any shit on him."

Being clean all the time was a luxury mostly only nobles and royalty had.