My pal and I rode loaded from Sandakan to Kuching last year. We first rode from Singapore to KLIA to get acclimatised. Although we are ancient, we're very fit, and managed the heat and humidity fairly well.
Accommodation and food was well spaced. The secret was a papaya for breakfast each morning, and a large beer in the evenings. The roads were generally excellent, and drivers well behaved towards cyclists, not that we saw many local ones.
I would like to organise an annual town to town cycle event to encourage Malaysians to get into long-distance cycling. If you are interested, Email me for further details and discussion.