VERY cool -- I hope you do this! I have been looking at fire trucks as well because I really would like a cabover type vehicle as I think they make the most sense for getting the most out over the 65 feet length limit you are allowed in overall combined vehicle + trailer length. With a conventional truck nose, you lose several feet of living/cargo space for the nose.
It isn't easy to find nice cabover type vehicles in the USA. They have tons of cool ones in Europe and Australia, including lots of 4WD units. Many of the emergency units in those places use the standard cab on them too.
The main hesitation I've had is wondering if it would be possible to cull enough weight off what surely must be a super heavy vehicle. And then not being sure what would be left or how clean it would be.
Another hesitation was not knowing how comfortable the cab is in these units compared to a fairly recent HDT or MDT.
PS... You might find this unit interesting that I recently came across on eBay. It's not an RV conversion.... just a tractor conversion. https://www.ebay.com/itm/114732514570