I have a 210 watt solar on my lithium battery..is it safe to also use a battery tender at the same time as the solar while my hauler is sitting at the house ?

 

It's surprising to me how rapidly mine will discharge. After sitting for a month and a half in the northwest USA both batteries were completely dead. They were two years old at the time.
The new batteries are less than two years old now and were "mostly dead" after sitting in my Texas garage for two months. I had added a dash cam which is continually powered and apparently this was enough to do the damage. I now disconnect it when stored in my garage.

 

From what I have read, unlike lead acid batteries Li batteries don't like to be stored at full charge. Stored at 50% to 80% charge may make them last longer. Below 20% may be too low. You might want to check and see if you find the same.
I am not sure if the chargers can be combined if you just want to bring a low battery up to full charge. Mine can be charged with both a DC/DC charger and solar and doesn't have any problems with being on shore power and solar is in use. It says the Solar has priority over the other ones. I have a renogy solar controler.

 

Yes, you can have multiple charging devices. However, when it comes to LiFePO4 batteries, you need chargers that are LiFePO4-aware. A traditional lead acid battery charger is not good for LiFePO4. If the charger has an AGM setting, that's usually fairly safe for LiFePO4.

My 560 Ah of LiFePO4 batteries stay in my trailer year round and get charged by solar. I use a Victron MPPT solar charge controller that knows how to charge LiFePO4. When not in use, I lower the charger settings. I'm in my trailer working on something year round, so disconnecting the batteries isn't an option. If I was going to store the trailer unattended for an extended period (like a year) I would discharge the LiFePO4 batteries to 70% and disconnect them for all loads and charges.

EDIT: The concern with multiple charge sources is that you don't want to exceed the maximum amps the LiFePO4 battery wants to see. The general rule of thumb is to limit charge/discharge to .5C for a good long life of the batteries.