Not sure on tank size or sump size so my next suggestion depends on this but if already got a ato set up and you say rodi reservoir only hold enough top up for about 1 weeks evaporation,then if possible you could drain some water out sump but ovniously enough to keep pump running then add larger rodi reservoir for top up then if ato sensor did fail and kept running,now you have extra space in your sump for the extra rodi water,ovbiously if ato fails and stops topping off then sump will go below return pump quicker and pump will run dry but above is option also.
But best bet in my opinion is turn temperature of tank down and dont run your sump and put heater in dt if needed and install a powerhead in dt for flow but don't aim powrrhead at surface therefore reducing surface agitation and therefore reducing evaporation and add some kind of lid so less heat,less surface agitation and lid will = less evaporation and bacteria really resident but if salinity rises a bit and bacteria go dormant or a few die or even if all dies then worse that will happen is just possible start your cycle again but there alot of depends in our answers as you not mentioning size of tank/ sump,size of top up reservoir,how long your holiday is for,how long tank need cycled for so far,plus lots of other info that could be useful.
All I do when go on holiday is fill up top up reservoir and make sure it's not higher water level than sump water level so a gravity siphon doesn't start and put food in bags and telly tank sitter to keep eye on things and tell them if top up reservoir gets really low and will run out before I get back to use some rodi water and unscrew lid on top up reservoir and use a funnel to put some more rodi water in and pit thermoter on front glass and tell them to keep eye on temperature and few other important things like powrrheads still breaking water surface and message me if some these important things are not how I want it then I will explain what to do on a video call ^_^
Edit: you could also drain sump and turn all equipment off in sump and put ato on dt Instead of sump and have larger ato and if ato did fail and stayed on then the extra rodi water would just flow down into sump drilling that up so wouldn't get flood from ato being left on ( ovbiously don't have such a large reservoir so could and ovbiously still would want powerhead in dt and maybe heater in dt.
Lots of ways you could work this situation but again only water and at most a bottle of bacteria in dt so of had start cycle again,not that big a deal in opinion ^_^
