We do a lot of Lithium conversions and our first priority is reducing the risk of battery theft. Losing a 70# $300 AGM is painful (especially for the thief!), but worse to lose a 30# $1000 Lithium. When we look at a conversion, here are our preferred methods of security
1) Mount the batteries inside the coach. Best security and good temperature control. Lithium has no fumes or acid, so no problem being under the bed. If you do this, make sure you use LiFePO4 technology, because it is safe from overheating and meltdown. Some other Lithium technologies are not.
2) If under the coach, we create locking brackets, often by welding or bolting on angle aluminum. We have even welded the brackets shut, since Lithium needs no maintenance for 10 years or more.
3) If on the tongue, we buy or build a secure aluminum box. E-trailer has them. We sometimes fabricate out of aluminum to meet custom sizes and shapes.
4) If in the front compartment or basement of a 5er, we add standard locking clasp latches to the doors. It is still easy to pry a door open with a pry bar, and in one case we reinforced the door and hinges. But clasp locks are a good visible deterrent.
5) If all else doesn't work, we use hardened security cable through the battery tie downs molded into the battery. This is also mostly just a visible deterrent, since plastic molded battery cases are easily cut.
Hope this is helpful.