Welcome to the forum.
The bolts are accessed from the top, there are 4 of them. While I haven't done this replacement myself, have read of several who have and they say the hardest part is getting the old unit off to the ground and the new one up on the roof.
Need to be careful of the replacement units due to the age of your trailer--need to verify with Dometic about the compatibility of the wiring and of the thermostats. Several of the posts mentioning replacing old units said they had communication problems with the thermostats.
Will need new lower gasket kit for sure. Take the A/C cover off and look around for the lag bolts that are screwed into the roof--probably hidden and/or covered by caulking.
I had a 36TK3 for 10 years and luckily never had to replace an A/C.
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired