You might want to watch a number of youtube videos and see if you have the skill set and tools to do this.
Not making a recommendations to DIY or not but I would think and stating the obvious that the critical safety items include turning off power and propane, ensuring good ventilation. Also making sure you have absolutely no propane leaks, exhaust leaks, or water leaks at the new connections. Any propane and exhaust leaks would be dangerous and potentially deadly. For home, WHs, new gas supply lines and valve are used - you’d have to check on best practice for RVs and not go low budget. You may need a new gasket or whatever is used at the exhaust. You’ll also need to know how your water supply/exit lines are done - ie PEX or other.
Many things aren’t rocket science but could be done wrong with significant consequences if one doesn’t have the aptitude, skills, tools, etc. I don’t mean that to sound harsh but underestimating the complexity can lead to problems.
The above is probably a few of the reasons a mfr would state it needs to be installed by a certified RV tech. Also, you might check if your warranty would be honored if you don’t use a certified RV tech.