The Greek text of John 14:2 according to the Textus Receptus (Stephanus, 1611) states:
ἐν τῇ οἰκίᾳ τοῦ πατρός μου μοναὶ πολλαί εἰσιν εἰ δὲ μή εἶπον ἂν ὑμῖν πορεύομαι ἑτοιμάσαι τόπον ὑμῖν
The word in question is μοναὶ (monai). It is the nominative plural declension of the feminine-gender, root noun μονή (monē). The noun μονή and its various declensions occur twice in the Greek text of the New Testament, both of which are translated into the Latin Vulgate by the Latin word mansiones, the plural, nominative/accusative declension of the root noun mansio. (I'm not sure why Jerome decided to translate John 14:23 into plural, since the Greek noun μονὴν is singular, but that seems to be the case.)
| Verse | TR | Vulgate | English Translation (KJV)
| John 14:2 | μοναὶ | mansiones | mansions
| John 14:23 | μονὴν | mansiones | abode
Now as you might know by now, the English word "mansion" is etymologically derived from the Latin word mansio.
According to Merriam-Webster,
Middle English, from Anglo-French, from Latin mansion-, mansio, from manēre to remain, dwell; akin to Greek menein to remain
Lewis and Short (p. 1109) define the Latin word mansio as,
I.a staying, remaining, stay, continuance.
I. Lit. (class.): “is saepe mecum de tua mansione, aut decessione communicat,” Cic. Fam. 4, 4, 5: “mansio Formiis,” id. Att. 9, 5, 1: “excessus e vita et in vita mansio,” id. Fin. 3, 18, 60: “cautior certe est mansio,” id. Att. 8, 15, 2: “diutinae Lemni,” Ter. Phorm. 5, 8, 23: crebrae ad amicam, i. e. visits, Turp. ap. Non. 132, 16.—
II. Transf. (post-Aug.), a place of abode, a dwelling, habitation.
A. In gen.: “pecorum mansio,” Plin. 18, 23, 53, § 194: “aestivae, hibernae, vernae, auctumnales,” Pall. 1, 9, 5; 1, 12: “mansionem apud eum faciemus,” Vulg. Joann. 14, 23: “multae mansiones,” id. ib. 14, 2.—
Night-quarters, lodging-place, inn; also, as a measure of days' journeys, a stopping or haltingplace, station: “deinde ad primam statim mansionem febrim nactus,” Suet. Tib. 10: “a quo (monte) octo mansionibus distat regio, etc.,” i. e. stations, days' journeys, Plin. 12, 14, 30, § 52: “aquationum ratione mansionibus dispositis,” id. 6, 23, 26, § 102: “continuatis mansionibus,” Just. 13, 8, 5.—
Mala mansio, bad quarters, a kind of punishment in which the culprit was stretched out and tied fast to a board, Dig. 47, 10, 15; 16, 3, 7.
As you can see from their definition, there is nothing that suggests something like this:
In fact, according to Merriam-Webster, one of the definitions of the English word "mansion" is "dwelling, abode." However, that definition is noted as "archaic," as the word is no longer used in that manner. Yet, the word "mansion" did possess that meaning when the King James Version was produced in 17th century England.
However, we shouldn't even be focusing on the Latin word mansio as the New Testament was originally composed in Greek. There is nothing about the Greek word μονή that suggests a luxurious estate either. According to Thayer, it simply means a "dwelling" or "abode."