There would be several ways, but the first three not really cheap.
1. Run your generator (which you should do about once a month, anyway) The converter will charge them slowly.
2. Run your engine on the MH which should charge your house batteries.
3. Use a jumper cable from another vehicle to your house batteries. - be really careful hooking up and unhooking, batteries generate hydrogen gas and is rather explosive.
The very best way is to plug into shore power and use a battery charger that has a trickle maintainer phase or have a 3 way converter that is like the WFCO that I have that does a maintainable level voltage - 13.5 or so and then #1 will work the best of the above three alternatives if you don't plug in to shore power.