Thanks for the input, even though it's given all the answers away! I checked all the answers with multiple sources (not just Google) and the use of RTGS was not promoted heavily. On the RTGS wiki page it even says "... its use, even by the government, is inconsistent." There are also several criticisms of the system as it doesn't transcribe all features of Thai phonology. I guess the sources I used took some of those criticisms into account.
You don't have to use RTGS, but Google translate's system is inaccurate and inconsistent (it doesn't take into account that consonants change sounds in different positions, and also uses x to denote a vowel sound for some reason). You can generate multiple different transcription systems using this website: http://www.thai-language.com/default.aspx.
ขอบคุณ - khopkhun (thank you)
ดอกไม้ - dok mai (flower)
ทะเล - thale (sea)
ขาว - si khao (white)
เดือน - due ron (summer)
น้ำ - nam (water)
ประเทศไทย - phra thet thai (Thailand)
ผม - phom (I, me)
ภูเขา - phukhao (mountain)
มือ - mue (hand)
ยินดีต้อนรับ - yin di thon rap (welcome)
รถไฟ - rot fai (train)
ร้านอาหาร - ran ahan (restaurant)
ฤดูร้อน - du ron (summer)
สัปปะรด - sapparot (pineapple)
สิบ - sip (ten)
หมา - ma (dog)
ห้องสมุด - hong samut (library)
โรงเรียน - rong rian (school)
โรงแรม - rong raem (hotel)