How much heat for how long to Kill RHDV?
Often, in studies of viral infection, heat-inactivated viral stocks are used as a non-infectious control.  See Matthaei et al. 2014 1 for an explanation of how such a stock was prepared for RHDV:

Heat-inactivated RHDV stock was generated by incubating the virus preparation in 2 ml screw cap tubes for 180 s at 85°C in a water bath to ensure a drop in infectious virus of more than 99.9%

The methods section for that paper cites a study by Duizer et al. 2 on the inactivation of caliciviruses (which include RHDV).  Therein, the authors tested the heat inactivation of canine calicivirus no. 48 (CaCV) and feline calicivirus F9 (FeCV) at 4, 20, 27, 37, 56, 63, 71.3, and 100°C for time periods varying from weeks (4°C) to seconds (71.3 and 100°C):


Times required at different temperatures to achieve a 3 log10 reduction in viable virus titer (TCID50) in DMEM (3 to 4 μg of protein/ml).

Note that, in both studies, viral heat inactivation is carried out in liquid culture, where it is easy to obtain a uniform temperature by mixing.  For heat inactivation of RHDV on food/hay, rate and uniformity of heating will be different.
Citations

Matthaei M, Kerr PJ, Read AJ, et al. Comparative quantitative monitoring of rabbit haemorrhagic disease viruses in rabbit kittens. Virol J. 2014;11:109.
Duizer E, Bijkerk P, Rockx B, De Groot A, Twisk F, Koopmans M. Inactivation of caliciviruses. Appl Environ Microbiol. 2004;70(8):4538-4543.