Decomposition reaction of tin(II) nitrate
No wonder you cannot find any literature on $\ce{Sn(NO3)2}$, because I don't think it exists in solid form. This is supported by the fact that I even can't find its CAS number online. This book (Ref.1) and relevant paper (Ref.2) support my suggestion, both of which state that:


  Attempts to prepare a covalently bound tin(II) nitrate, by the reaction of tin(IV) tetranitrate with anhydrous nitric oxide, have produced only a white solid solid of formula $\ce{SnN2O6}$, which gives a tin(IV) Mössbauer resonance ($\delta = \pu{0.29 mm\:s-1}$, $\Delta = \pu{0.96 mm\:s-1}$)...


Nonetheless, a study on thermal decomposition of metal nitrates (Ref.3) states that:


  Due to a back-donation of electronic cloud from the nitrate to an unfilled $\mathrm{d}$-orbital of transition and noble metals, their nitrates generally exhibited lower decomposition temperatures ($T_d \lt \pu{700 K}$) than those of the base metals ($\gt \pu{850 K}$).


They suggest those metal nitrates with lower decomposition temperatures decompose to corresponding oxides together with $\ce{NO2}$ and $\ce{O2}$. Thus, it is safe to assume that if $\ce{Sn(NO3)2}$ exists, it would decompose as following reaction suggest:

$$\ce{2 Sn(NO3)2 (s) + heat -&gt; 2 SnO (s) + 4 NO2 (g) + O2 (g)}$$

However, keep in mind that this is pure speculation.

References:


R. Greatrex, “Chapter 8: Mössbauer Spectroscopy,” In Specialist Periodical Reports: Spectroscopic Properties of Inorganic and Organometallic Compounds, Volume 6; N. N. Greenwood, Ed.; The Chemical Society Burlington House: London, United Kingdom, 1973, pp. 494-622.
P. G. Harrison, M. I. Khalil, N.Logan, “Concerning anhydrous tin(II) nitrate,” Inorganic and Nuclear Chemistry Letters 1972, 8(6), 551-553 (https://doi.org/10.1016/0020-1650(72)80139-9).
Shanmugam Yuvaraj, Lin Fan-Yuan, Chang Tsong-Huei, Yeh Chuin-Tih, “Thermal Decomposition of Metal Nitrates in Air and Hydrogen Environments,” J. Phys. Chem. B 2003, 107(4), 1044-1047 (https://doi.org/10.1021/jp026961c).